Dadas as matrizes $A=\left[\begin{array}{cc}1& 0\\ -1& -1\\ 1& 1\end{array}\right]$ e $B=\left[\begin{array}{ccc}0& 1& 2\\ 3& 4& 5\end{array}\right]$ e considerando $n=\det (AB)$, determine $7^n$.