This is a solution to the first equation, but not to the system. If you substitute these values into the second equation you get $\displaystyle{189}+{66}=-{31}\usepackage{cmbright}\Huge\$ , which is false. Solve the first equation for x: $\displaystyle{x}={16}-{y}\usepackage{cmbright}\Huge\$ . Then substitute $\displaystyle{16}-{y}\usepackage{cmbright}\Huge\$ for x in the second equation and solve for y. Substitute this value into any equation to find x.