You probably forgot that $\displaystyle{c}={{c}}^{{1}}\usepackage{cmbright}\Huge\$ , and treated this factor as if the exponent was 0. Use the power rule first to simplify $\displaystyle{{\left({{c}}^{{4}}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ , and then use the product rule.