You made a mistake with a sign when you were combining $\displaystyle-{{x}}^{{5}}+{2}{{x}}^{{5}}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{{x}}^{{5}}+{6}{{x}}^{{4}}-{6}{{x}}^{{3}}\usepackage{cmbright}\Huge\$ .