Do as directed
Regards

Dear student, [\begin{matrix} 1 & 2 & - 1 \\ 0 & 4 & 9 \end{matrix}] = [\begin{matrix} 9 & - 1 & 4 \\ - 2 & 1 & 3 \end{matrix}] - A A = [\begin{matrix} 9 & - 1 & 4 \\ - 2 & 1 & 3 \end{matrix}] - [\begin{matrix} 1 & 2 & - 1 \\ 0 & 4 & 9 \end{matrix}] A = [\begin{matrix} 8 & - 3 & 5 \\ - 2 & - 3 & - 6 \end{matrix}] Regards