While this expression has been simplified correctly, it is not in simplest form. Simplify the second radical by using $\displaystyle{\sqrt[{{3}}]{{{a}{b}}}}={\sqrt[{{3}}]{{{a}}}}\cdot{\sqrt[{{3}}]{{{b}}}}\usepackage{cmbright}\Huge\$ . You should end up with the same radicands, which means that you have like terms. Then you can combine them just as you would combine $\displaystyle{2}{a}+{3}{a}\usepackage{cmbright}\Huge\$ .