It appears that you subtracted the radicands ( $\displaystyle{20}-{5}\usepackage{cmbright}\Huge\$ ). 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 term 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.