Item 1.2.5.1.c
\begin{equation*}
\begin{alignedat}{5}
x_1 \amp {}+{} \amp 2x_2 \amp {}-{} \amp 5x_3 \amp
{}-{} \amp x_4 \amp {}={} \amp -3 \\
-2x_1 \amp {}-{} \amp 2x_2 \amp {}+{} \amp 6x_3 \amp
{}-{} \amp 2x_4 \amp {}={} \amp 4 \\
x_1 \amp \amp \amp {}-{} \amp x_3 \amp {}+{}
\amp 9x_4 \amp {}={} \amp 7 \\
\amp \amp -x_2 \amp {}+{} \amp 2x_3 \amp {}-{}
\amp x_4 \amp {}={} \amp 4 \\
\end{alignedat}
\end{equation*}