已知 $\left\{\begin{array}{c}x=2\\ y=-3\end{array}\right.$是方程组 $\left\{\begin{array}{c}\mathit{ax}+\mathit{by}=2\\ \mathit{bx}+\mathit{ay}=3\end{array}\right.$的解，则 $a^2-b^2=$　　．