a
(a) $A = \left[ {\begin{array}{*{20}{c}}1&2&{ - 1}\\3&0&2\\4&5&0\end{array}} \right]$, $B = \left[ {\begin{array}{*{20}{c}}1&0&0\\2&1&0\\0&1&3\end{array}} \right]$
$AB = \left[ {\begin{array}{*{20}{c}}{1 \times 1 + 2 \times 2 + ( - 1)(0)}\\{3 \times 1 + 0 \times 2 + 2 \times 0}\\{4 \times 1 + 5 \times 2 + 0 \times 0}\end{array}} \right.$
$\left. {\begin{array}{*{20}{c}}{1 \times 0 + 2 \times 1 + ( - 1)\,(1)}\\{3 \times 0 + 0 \times 1 + 2 \times 1}\\{4 \times 0 + 5 \times 1 + 0 \times 1}\end{array}\begin{array}{*{20}{c}}{\,\,\,\,1 \times 0 + 2 \times 0 + ( - 1)\,(3)}\\{3 \times 0 + 0 \times 0 + 2 \times 3}\\{4 \times 0 + 5 \times 0 + 0 \times 3}\end{array}} \right]$
$\therefore \,\,AB = \left[ {\begin{array}{*{20}{c}}5&1&{ - 3}\\3&2&6\\{14}&5&0\end{array}} \right]$.