You probably thought that you had to find the opposite of the first 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 you would get $\displaystyle-{5}{x}+{3}{y}=-{5}{\left(-{5}\right)}+{3}{\left({15}\right)}={25}+{45}={70}\usepackage{cmbright}\Huge\$ , not 20.