You may have simplified the radicals correctly, but then tried to cancel a factor of $\displaystyle{\sqrt[{{3}}]{{{2}{{y}}^{{2}}}}}\usepackage{cmbright}\Huge\$ with the term $\displaystyle-{\sqrt[{{3}}]{{{2}{{y}}^{{2}}}}}\usepackage{cmbright}\Huge\$ . You combine like terms by adding or subtracting their coefficients, not by canceling their variables, $\displaystyle{2}{a}-{a}={2}{a}-{1}{a}={\left({2}-{1}\right)}{a}={a}\usepackage{cmbright}\Huge\$ .