Consider the following problem: find $n(A \cap B)$ if $n(A)=10$, $n(B)=13$ and $n(A \cup B) = 15$.

I know if I want to find the union I use the Cardinal Number formula:

$$n(A\cup B) = n(A) + n(B) - n(A\cap B)$$ 

But how do I do it the other way: to find $n(A\cap B)$? 

Would it be $n(A\cap B) = n(A) + n(B) - n(A\cup B)$ ?