You may not have noticed the index of the cube root, and so incorrectly combined the first two radicals, while not simplifying the third one. Start by simplifying the last term using $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{a}}\cdot\sqrt{{b}}\usepackage{cmbright}\Huge\$ . Now the first and last terms are like terms that you can combine, just as you would combine $\displaystyle{9}{a}+{2}{a}\usepackage{cmbright}\Huge\$ .