You may have multiplied the first equation by 2 instead of by $\displaystyle-{2}\usepackage{cmbright}\Huge\$ and made a mistake combining it with the second equation. This ordered pair satisfies the first equation, but not the second. If you substitute it into the left side of the second equation you will get $\displaystyle{3}{x}+{2}{y}={3}{\left({4}\right)}+{2}{\left({1}\right)}={12}+{2}={14}\usepackage{cmbright}\Huge\$ , not 10.