You made a mistake with a minus sign at some point in your solution. You should be solving two equations, $\displaystyle{3}{c}+{6}={12}\usepackage{cmbright}\Huge\$ and $\displaystyle{3}{c}+{6}=-{12}\usepackage{cmbright}\Huge\$ . To solve these equations, you need to subtract 6 from both sides as your first step.