Answer
$n(A\cap B)[n(A\cup B)-n(A')]=2$
Work Step by Step
$n(A\cap B)$ is the number of elements of $A$ that are also elements of $B$.
We can see that $n(A\cap B)=2$
$n(A\cup B)$ is the number of distinct elements which are in $A$, in $B$, or in both $A$ and $B$.
We can see that $n(A\cup B)=7$
n(A') is the number of elements in the diagram that are not in $A$.
We can see that $n(A')=6$
Therefore:
$n(A\cap B)[n(A\cup B)-n(A')]=2*(7-6)=2$
