This ordered pair is a solution for the second equation, but not for the first equation. If you substituted into the left side of the first equation you would get $\displaystyle-{2}{x}+{y}=-{2}{\left(-{3}\right)}+{6}={12}\usepackage{cmbright}\Huge\$ , not $\displaystyle-{8}\usepackage{cmbright}\Huge\$ .