You may have mistakenly combined the unlike terms $\displaystyle{6}{{x}}^{{4}}\usepackage{cmbright}\Huge\$ and $\displaystyle-{6}{{x}}^{{3}}\usepackage{cmbright}\Huge\$ . Group the like terms together: $\displaystyle{\left(-{{x}}^{{5}}+{2}{{x}}^{{5}}\right)}+{6}{{x}}^{{4}}-{6}{{x}}^{{3}}+{\left({1}-{1}\right)}\usepackage{cmbright}\Huge\$ . Then simplify. The correct answer is $\displaystyle{{x}}^{{5}}+{6}{{x}}^{{4}}-{6}{{x}}^{{3}}\usepackage{cmbright}\Huge\$ .