已知矩阵 $\left[\begin{array}{ll}a & 1 \\ a & 0\end{array}\right] \cdot\left[\begin{array}{cc}b & 1 \\ 0 & b^2\end{array}\right]=\left[\begin{array}{ll}6 & 7 \\ 6 & 3\end{array}\right]$ ，求常数 $a, b$ ．
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