You may have multiplied and simplified correctly, but then simplified beyond what was possible by adding the radicands. Though you can simplify using $\displaystyle\sqrt{{a}}\cdot\sqrt{{b}}=\sqrt{{{a}{b}}}\usepackage{cmbright}\Huge\$ , you cannot simplify $\displaystyle\sqrt{{a}}+\sqrt{{b}}\usepackage{cmbright}\Huge\$ as $\displaystyle\sqrt{{{a}+{b}}}\usepackage{cmbright}\Huge\$ .