You may have correctly multiplied the first equation by $\displaystyle-{4}\usepackage{cmbright}\Huge\$ and combined the equations to find that $\displaystyle{x}={15}\usepackage{cmbright}\Huge\$ , but then mixed up the variables. 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({2}\right)}+{4}{\left({15}\right)}={6}+{60}={66}\usepackage{cmbright}\Huge\$ , not 1.