You multiplied $\displaystyle{\left({6}{{x}}^{{4}}+{4}{{x}}^{{2}}+{12}\right)}\usepackage{cmbright}\Huge\$ by $\displaystyle-{x}\usepackage{cmbright}\Huge\$ , not by $\displaystyle{\left(\frac{{1}}{{2}}{{x}}^{{2}}-{x}\right)}\usepackage{cmbright}\Huge\$ .