You may have simplified the second radical and then tried to combine all three terms. You can combine the first and third terms. This is because they are both multiples of $\displaystyle\sqrt{{x}}\usepackage{cmbright}\Huge\$ , which makes them like terms. The second term is a multiple of $\displaystyle{\sqrt[{{3}}]{{{x}}}}\usepackage{cmbright}\Huge\$ , so it is an unlike term.