You may have correctly multiplied the first equation by $\displaystyle-{2}\usepackage{cmbright}\Huge\$ and combined the equations to find that $\displaystyle{y}={2}\usepackage{cmbright}\Huge\$ , but then mixed up the variables. This ordered pair satisfies the second equation, but not the first. If you substitute it into the left side of the first equation you will get $\displaystyle{x}-{2}{y}={2}-{2}{\left({0}\right)}={2}-{0}={2}\usepackage{cmbright}\Huge\$ , not $\displaystyle-{5}\usepackage{cmbright}\Huge\$ .