You may have incorrectly combined the two radicals in the parentheses by subtracting the radicands to obtain $\displaystyle{\left(\sqrt{{{18}}}\right)}{\left(\sqrt{{{16}}}\right)}\usepackage{cmbright}\Huge\$ . Instead you should use the Distributive Property and multiply each term in the parentheses by the radical outside the parentheses. Use $\displaystyle\sqrt{{a}}\cdot\sqrt{{b}}=\sqrt{{{a}{b}}}\usepackage{cmbright}\Huge\$ . Then simplify each radical.