Find the product $AB,$ where $(5$ points each$)$

a$)A=\left[\begin{array}{ccc}1& 0& 1\\ 0& -1& -1\\ -1& 1& 0\end{array}\right],B=\left[\begin{array}{ccc}0& 1& -1\\ 1& -1& 0\\ -1& 0& 1\end{array}\right]$