You may have thought that the square root of $\displaystyle-{9}\usepackage{cmbright}\Huge\$ is $\displaystyle-{3}\usepackage{cmbright}\Huge\$ , but it is not. The square root of $\displaystyle-{9}\usepackage{cmbright}\Huge\$ is equal to $\displaystyle{3}{i}\usepackage{cmbright}\Huge\$ , which is a purely imaginary number.