You may have tried to combine the terms by putting the subtraction under one radical sign, then simplifying further. When you multiply or divide radicals, you can combine them under one sign, but $\displaystyle{\sqrt[{{3}}]{{a}}}-{\sqrt[{{3}}]{{b}}}\usepackage{cmbright}\Huge\$ cannot be combined as $\displaystyle{\sqrt[{{3}}]{{{a}-{b}}}}\usepackage{cmbright}\Huge\$ . Instead, simplify the terms using $\displaystyle{\sqrt[{{3}}]{{{a}{b}}}}={\sqrt[{{3}}]{{a}}}\cdot{\sqrt[{{3}}]{{b}}}\usepackage{cmbright}\Huge\$ . Then look for like terms that can be combined.