\[ \left[ \begin{array}{c}
x \\
y \\ \end{array} \right] \] = \frac{1}{b(1+a)} \[ \left[ \begin{array}{cc}
b &  b \\
a  & -1 \\ \end{array} \right] \]  \[ \left[ \begin{array}{c}
0 \\
b \\ \end{array} \right] \]