If $ A = \begin{pmatrix} 4 & 2 \\ -1 & 1 \end{pmatrix} $ and $ I = \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} $ then prove that `(A-2I)(A-3I)=`$\begin{pmatrix} 0 & 0 \\ 0 & 0 \end{pmatrix}$