You probably did the correct first step, $\displaystyle{40}+{5}{{d}}^{{3}}={5}{\left({8}+{{d}}^{{3}}\right)}\usepackage{cmbright}\Huge\$ , but then made a mistake. 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\$ . The second factor in this answer is missing its middle term. Check your answer by multiplying out.