a
(a) $\left[ {\begin{array}{*{20}{c}}7&1&2\\9&2&1\end{array}} \right]\,\left[ {\begin{array}{*{20}{c}}3\\4\\5\end{array}} \right]\, = \,\,\left[ {\begin{array}{*{20}{c}}{35}\\{40}\end{array}} \right]$;
$\therefore$ $\left[ {\begin{array}{*{20}{c}}{35}\\{40}\end{array}} \right]\, + \,\left[ {\begin{array}{*{20}{c}}8\\4\end{array}} \right]\, = \,\left[ {\begin{array}{*{20}{c}}{43}\\{44}\end{array}} \right]$.