Do as directed
Regards

Dear student, [\begin{matrix} 2 & 1 & 3 \end{matrix}] [\begin{matrix} - 1 & 0 & - 1 \\ - 1 & 1 & 0 \\ 0 & 1 & 1 \end{matrix}] [\begin{matrix} 1 \\ 0 \\ - 1 \end{matrix}] = [\begin{matrix} - 2 - 1 + 0 & 0 + 1 + 3 & - 2 + 0 + 3 \end{matrix}] [\begin{matrix} 1 \\ 0 \\ - 1 \end{matrix}] = [\begin{matrix} - 3 & 4 & 1 \end{matrix}] [\begin{matrix} 1 \\ 0 \\ - 1 \end{matrix}] = [- 3 + 0 - 1] = [- 4] Regards