Similar Triangles--Find the measurement of the unknown side This is a question I know I got wrong on a final exam in a very easy class for teaching elementary geometry/prep for Praxis II. I actually received a 99% average in the entire course because of the single point taken off of the final. I just want to know how to go about solving this. I feel like I overthought the question. It was a multiple choice question and none of the answers seemed to make any sense to me. The question was to find the measurement of line segment CD.

Don't quote me on this, but I believe the choices were:
A.) 5 cm 
B.) 16 cm 
C.) 19 cm 
D.) 25 cm 
Either way. We don't really need the choices to answer the question. I just want to know how to go about solving this. I know it has to do with similar triangles and creating a super easy proportion, but I tried every method I could think of. None of my answers were close to what the multiple choice answers were. So, I obviously did something wrong. 

In case it's hard to see, AD=18cm AB= 7cm BC= 6cm. Triangle is not drawn to scale

Hint: Note that the sum of any two sides of a triangle is greater than the third side. That will be enough to eliminate $3$ of the given choices.
For example, $CD$ cannot be $5$, since $7+(6+5)$ is not greater than $18$.
