Item1.2.5.1.c

\begin{equation*} \begin{alignedat}{5} -2x_1 \amp {}+{} \amp 3x_2 \amp \amp \amp {}+{} \amp 2x_4 \amp {}={} \amp 7 \\ -x_1 \amp {}+{} \amp 4x_2 \amp {}+{} \amp x_3 \amp {}+{} \amp 3x_4 \amp {}={} \amp 9 \\ 3x_1 \amp \amp \amp {}+{} \amp 2x_3 \amp {}+{} \amp 2x_4 \amp {}={} \amp -1 \\ x_1 \amp {}+{} \amp 5x_2 \amp {}+{} \amp 4x_3 \amp {}+{} \amp 7x_4 \amp {}={} \amp 8 \\ \end{alignedat} \end{equation*}
in-context