Inclusion-exclusion principle: Number of integer solutions to equations The problem is:
Find the number of integer solutions to the equation
$$
x_1 + x_2 + x_3 + x_4 = 15
$$
satisfying
$$
\begin{align}
2 \leq &x_1 \leq 4, \\
-2 \leq &x_2 \leq 1, \\
 0 \leq &x_3 \leq 6, \text{ and,} \\
 3 \leq &x_4 \leq 8 \>.
\end{align}
$$
I have read some papers on this question, but none of them explain clearly enough. I am especially confused when you must decrease the total amount of solutions to the equation—with no regard to the restrictions—from the solutions that we don't want. How do we find the intersection of the sets that we don't want? Either way, in helping me with this, please explain this step.

I'm not sure I can expand on PEV's hints in a comment, so I'll make it an answer. 
You need to know the number of solutions of $$u_1+u_2+\dots+u_r=n$$ when the only restriction on the variables is that they be non-negative integers. Imagine $n+r-1$ dots in a line, and circle $r-1$ of them. The uncircled dots are $n$ in number, and the circled ones divide the uncircled ones into $r$ groups (some of which may be empty), so you get $r$ non-negative integers adding up to $n$. So the question becomes, how many ways can you choose which $r-1$ of the $n+r-1$ dots to circle? Unfortunately, PEV wrote 18-choose-3, where I think what's wanted is 15-choose-3, but now you should see how to get that part of the answer. 
Then you ask how to use inclusion-exclusion. It isn't clear whether you mean that you don't see how to get a formula for the size of the union by using inc-excl, or whether you mean that you can write down a formula but don't see how to find the sizes of the $A_i$ and the various intersections that arise, so it's a little hard to help you here. I'll assume it's the second suggestion. So for PEV's $A_1$, let $v_1=y_1-3$, then you have $v_1+y_2+y_3+y_4=9$ and the variables are non-negative, so the previous paragraph applies. Similarly, for the intersection of $A_1$ and $A_2$, let $v_1=y_1-3$ and $v_2=y_2-4$, so you get $v_1+v_2+y_3+y_4=5$ with all variables non-negative. 
Can you take it from there?
