First identify the like terms. Then combine them by adding coefficients.

$\displaystyle-{{x}}^{{5}}+{2}{{x}}^{{5}}+{6}{{x}}^{{4}}-{6}{{x}}^{{3}}+{{x}}^{{0}}-{1}={\left(-{1}+{2}\right)}{{x}}^{{5}}+{6}{{x}}^{{4}}-{6}{{x}}^{{3}}+{\left({1}-{1}\right)}={{x}}^{{5}}+{6}{{x}}^{{4}}-{6}{{x}}^{{3}}\usepackage{cmbright}\Huge\$