You may have mixed up the variables in one of the two equations. You should be starting with the equations: $\displaystyle{12}{S}+{4}{C}={42}\usepackage{cmbright}\Huge\$ and $\displaystyle{4}{S}+{12}{C}={30}\usepackage{cmbright}\Huge\$ . Multiply the first equation by $\displaystyle-{3}\usepackage{cmbright}\Huge\$ and add this to the second equation.