You may have mixed up the variables in one of the two equations. You should be starting with the two equations: $\displaystyle{15}{p}+{5}{c}={6}\usepackage{cmbright}\Huge\$ and $\displaystyle{5}{p}+{15}{c}={8}\usepackage{cmbright}\Huge\$ . Multiply the first equation by $\displaystyle-{3}\usepackage{cmbright}\Huge\$ and add this to the second equation.