已知 $\left\{\begin{array}{c}x=3\\ y=-2\end{array}\right.$是方程组 $\left\{\begin{array}{c}\mathit{ax}+\mathit{by}=3\\ \mathit{bx}+\mathit{ay}=-7\end{array}\right.$的解，则代数式 $(a+b)(a-b)$的值为　　．