Calcule os determinantes:

$$A=\left|\begin{array}{cll}1& a& 0\\ 0& 1& 1\\ 0& -1& 1\end{array}\right|\text{e}B=\left|\begin{array}{ccll}1& 0& 0& 3\\ a& 1& -1& 4\\ 0& 0& 0& 3\\ 0& 0& 1& 4\end{array}\right|$$