\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*}