You may have correctly multiplied the first equation by 4 and combined the equations to find that $\displaystyle{x}={6}\usepackage{cmbright}\Huge\$ , but then mixed up the variables. 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({28}\right)}+{6}={84}+{6}={90}\usepackage{cmbright}\Huge\$ , not 13.