$\displaystyle{7}{{a}}^{{3}}{{b}}^{{6}}{\left({5}{a}{{b}}^{{2}}+{2}\right)}={35}{{a}}^{{4}}{{b}}^{{8}}+{14}{{a}}^{{3}}{{b}}^{{6}}\ne{35}{{a}}^{{2}}{{b}}^{{4}}+{14}{{a}}^{{3}}{{b}}^{{6}}\usepackage{cmbright}\Huge\$ so this factorization is incorrect.