Find the values of a and b for which

Find the values of $a$ and $b$ for which

$\left[\begin{array}{cc}\mathrm{a} & \mathrm{b} \\ -\mathrm{a} & 2 \mathrm{~b}\end{array}\right]\left[\begin{array}{c}2 \\ -1\end{array}\right]=\left[\begin{array}{l}5 \\ 4\end{array}\right]$