The domain will exclude values for which the value of the denominator is 0. Factoring the denominator $\displaystyle{{x}}^{{2}}-{x}-{2}\usepackage{cmbright}\Huge\$ as $\displaystyle{\left({x}+{1}\right)}{\left({x}-{2}\right)}\usepackage{cmbright}\Huge\$ , you found that the value of the denominator will be 0 when $\displaystyle{x}=-{1}\usepackage{cmbright}\Huge\$ and when $\displaystyle{x}={2}\usepackage{cmbright}\Huge\$ . These values are excluded from the domain.