You treated the negative sign as if it were outside the radical and then found the square root of 81. However, the square root of a negative number is not real. 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.