Sets And Relations

Let $A$ and $B$ be two subsets of $\xi=\{\mathbf{1}, \mathbf{2}, \mathbf{3},-------, \mathbf{4 4}, \mathbf{4 5}\}$ such that

$A=\{x: x$ is divisible by 3 and 4$\}$

$B=\{x: x$ is a perfect square number $\}$

Then $n(B-A)$ equals