You may have used $\displaystyle-{9}\usepackage{cmbright}\Huge\$ as the square root of $\displaystyle-{81}\usepackage{cmbright}\Huge\$ , but that is not correct because $\displaystyle{\left(-{9}\right)}\cdot{\left(-{9}\right)}\usepackage{cmbright}\Huge\$ does not equal $\displaystyle-{81}\usepackage{cmbright}\Huge\$ . 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.