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