You may have tried to combine the first and third 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{{a}}-\sqrt{{b}}\usepackage{cmbright}\Huge\$ cannot be combined as $\displaystyle\sqrt{{{a}-{b}}}\usepackage{cmbright}\Huge\$ . Instead, simplify the first and third terms using $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{a}}\cdot\sqrt{{b}}\usepackage{cmbright}\Huge\$ . Then you will end up with like terms that can be combined.