You correctly factored out a 3, but then made a mistake with the signs. A binomial in the form $\displaystyle{{a}}^{{3}}+{{b}}^{{3}}\usepackage{cmbright}\Huge\$ can be factored as $\displaystyle{\left({a}+{b}\right)}{\left({{a}}^{{2}}-{a}{b}+{{b}}^{{2}}\right)}\usepackage{cmbright}\Huge\$ .