You may have multiplied the first equation by 4 instead of by $\displaystyle-{4}\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 you will get $\displaystyle{3}{x}+{4}{y}={3}{\left({7}\right)}+{4}{\left({5}\right)}={21}+{20}={41}\usepackage{cmbright}\Huge\$ , not 1.