I received an allegation of academic misconduct from my university. What should I do? I wrote an exam a couple of months ago and I recently received an email accusing me of academic misconduct. I had just finished my first year of university and this is the first time that I am receiving a letter of this kind.
One of the long answers that I had written matched an answer that was posted online on a homework solutions website.
Now due to Covid-19, this exam was an online open-book exam, I had interpreted the open book exam to mean that we could have our notes and our lecture slides with us. The weeks leading to the exam, I had searched up practice questions online and had written and solved them out on paper to have with me during the exam just in case a similar question came up.
Sure enough one of the practice questions that I had was very similar to the one on the exam. Thus, I had an easy time answering it and pretty much reused what I had written on my notes. I don't think that's cheating if I only looked at my notes on an open book exam. Also, the exam had a lot of math calculations and conclusion statements to go with it. Most questions like this have similar steps/conclusions.
I had contacted a student advocate to help me with my case and I'm supposed to write an opening statement to present to a meeting with the dean of the department.
The evidence against me seems very strong. If I am found guilty, I would get an F on the class and be suspended from taking courses for the months of May-August. But, I am already in the process of finishing 4-5 classes this summer. I am distressed that all of my money and hard work would all be wasted.
Is my evidence sufficient? What other evidence could I use to prove that I didn't commit academic misconduct? What should I write on my opening statement?
Clarifications:

I sifted through his emails regarding the final, and this was what he had written: &quot;Open-Book final exam. Despite the final exam being open-book you still need to be well prepared otherwise- you will not have enough time to finish the exam if you spend a lot of time looking through your notes.&quot; No other explanations were provided. Perhaps it was my fault that I didn't ask for clarifications and just assumed.
My notes and the solutions were very similar, but the conclusion was different. I had tried to solve the problem on my own, but referenced the solution when I got stuck.
In a comment, you write that the professor had written in an email regarding the exam:

you will not have enough time to finish the exam if you spend a lot of time looking through your notes

This email is absolutely key. Quote the full email in your statement (including the date and timestamp), and highlight this sentence. It immediately implies that you were assumed and allowed to use notes.
The next point is that the professor apparently did not place any restrictions on what could be in your notes. (Did they?) Point this out.
The next exhibit should be that is was apparently easy to find practice questions that turned out to be similar to questions on the exam.
Nobody can hold it against you if you used the internet in researching what might come up in the exam, and in preparing your notes. (Yes, it would be better to reformulate your notes in your own words. Do this next time. You learn more that way.)
That should already be enough of an argument to rebut the accusation. Here is the structure of the argument I would propose:

You were allowed to use notes. Quote the email.
No restrictions on the notes were given.
You worked practice questions from various sources, e.g., from the internet. Quote the practice question, include if possible the link and a screenshot.
For one practice question, you used an answer you found online and put this in your notes. Again, include the link to the answer and a screenshot. Possibly include a copy of your notes.
One question on the exam was very similar to this practice question. Quote the question on the exam, perhaps highlight the similarities.
Therefore, it is not surprising that your answer matches something found in the internet.

Write this up in a short succinct way. Use paragraph breaks between the arguments. Don't ramble. Make this a short document of two pages at most.
If there is any sanity at your school, this accusation will be dropped immediately, and the professor will get some friendly advice about what to expect if they use questions on the exam that match questions that diligent students might find online while preparing.