This expression, though simplified corrrectly, is not in simplest form. Simplify the second radical by using $\displaystyle{\sqrt[{{4}}]{{{a}{b}}}}={\sqrt[{{4}}]{{{a}}}}\cdot{\sqrt[{{4}}]{{{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\$ .