You probably thought that you had to find the opposite of the second equation before adding the equations. This ordered pair is a solution for the first equation, but not for the second equation. If you substituted into the left side of the second equation you would get $\displaystyle{x}-{4}{y}={7}-{4}{\left(-{3}\right)}={7}+{12}={19}\usepackage{cmbright}\Huge\$ , not $\displaystyle-{5}\usepackage{cmbright}\Huge\$ .