a
(a)${A^2} = A\,.\,A = \left[ {\begin{array}{*{20}{c}}{\rm{2}}&{ - 1}\\{ - 1}&2\end{array}} \right]\,\left[ {\begin{array}{*{20}{c}}{\rm{2}}&{ - 1}\\{ - 1}&2\end{array}} \right] = \left[ {\begin{array}{*{20}{c}}{\rm{5}}&{ - 4}\\{ - 4}&5\end{array}} \right]$
==> $4A - 3I = \,\left[ {\begin{array}{*{20}{c}}{\rm{8}}&{ - 4}\\{ - 4}&8\end{array}} \right]\, - \,\left[ {\begin{array}{*{20}{c}}{\rm{3}}&0\\0&3\end{array}} \right] = \left[ {\begin{array}{*{20}{c}}5&{ - 4}\\{ - 4}&5\end{array}} \right]$.