Item1.2.5.1.a

\begin{equation*} \begin{alignedat}{3} 2x \amp {}+{} \amp y \amp {}={} \amp 0 \\ x \amp {}+{} \amp 2y \amp {}={} \amp 3 \\ -2x \amp {}+{} \amp 2y \amp {}={} \amp 6 \\ \end{alignedat} \end{equation*}
in-context