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 second equation, but not the first. If you substitute it into the left side of the first equation you will get $\displaystyle{2}{x}+{y}={2}{\left({4}\right)}+{\left(-{1}\right)}={8}+{\left(-{1}\right)}={7}\usepackage{cmbright}\Huge\$ , not 9.