$\left[ {\begin{array}{*{20}{c}}
x&y \\
2&{ - 1}
\end{array}} \right]\left[ {\begin{array}{*{20}{c}}
4&3 \\
3&4
\end{array}} \right] = \left[ {\begin{array}{*{20}{c}}
4&3 \\
3&4
\end{array}} \right]\left[ {\begin{array}{*{20}{c}}
x&y \\
2&{ - 1}
\end{array}} \right]$
$ \to \left[ {\begin{array}{*{20}{c}}
{4x + 3y}&{3x + 4y} \\
5&2
\end{array}} \right] = \left[ {\begin{array}{*{20}{c}}
{4x + 6}&{4y - 3} \\
{3x + 8}&{3y - 4}
\end{array}} \right]$
$3x + 8 = 5 \to x = - 1\begin{array}{*{20}{c}}
{}&,
\end{array}\begin{array}{*{20}{c}}
{}&{3y - 4 = 2 \to y = 2}
\end{array}$
$\left[ {\begin{array}{*{20}{c}}
{ - 1}&2&{ - 2}
\end{array}} \right]\left[ {\begin{array}{*{20}{c}}
2 \\
2 \\
1
\end{array}} \right] = - 2 + 4 - 2 = 0$