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 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(-{10}\right)}-{2}=-{20}-{2}=-{22}\usepackage{cmbright}\Huge\$ , not 18.