Groups and the Group Axioms · Hard problem

The $2\times2$ matrices $\begin{pmatrix}a&b\\0&1\end{pmatrix}$ with $a\in\{1,-1\}$ and $b\in\mathbb Z$ form a group under matrix multiplication. For $A=\begin{pmatrix}-1&2\\0&1\end{pmatrix}$ and $B=\begin{pmatrix}1&3\\0&1\end{pmatrix}$, find (top-right entry of $AB$) $-$ (top-right entry of $BA$).