You made a mistake with a sign when you were combining $\displaystyle-{{x}}^{{5}}+{2}{{x}}^{{5}}\usepackage{cmbright}\Huge\$ and you incorrectly simplified the term $\displaystyle{{x}}^{{0}}\usepackage{cmbright}\Huge\$ . Any number to the zero power equals 1. The correct answer is $\displaystyle{{x}}^{{5}}+{6}{{x}}^{{4}}-{6}{{x}}^{{3}}\usepackage{cmbright}\Huge\$ .