You may have found the correct answer, but then tried to simplify further by multiplying the number outside the radical by the number under the radical, which is not correct. Use the rule $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{{a}}}\sqrt{{{b}}}\usepackage{cmbright}\Huge\$ and the definition $\displaystyle{i}=\sqrt{{-{1}}}\usepackage{cmbright}\Huge\$ to simplify.