You may have correctly simplified each term by using $\displaystyle{\sqrt[{{3}}]{{{a}{b}}}}={\sqrt[{{3}}]{{{a}}}}\cdot{\sqrt[{{3}}]{{{b}}}}\usepackage{cmbright}\Huge\$ , but then incorrectly multiplied the coefficients of the radicals. After simplifying each term, 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\$ .