You may have multiplied the first equation by $\displaystyle-{4}\usepackage{cmbright}\Huge\$ instead of by 4 and made a mistake combining it with the second equation. This ordered pair satisfies the second equation, but not the first. If you substitute it into the left side you will get $\displaystyle{3}{x}+{y}={3}{\left({2}\right)}-{7}={6}-{7}=-{1}\usepackage{cmbright}\Huge\$ , not 13.