You may have thought that the two negative signs canceled, leaving you with the real number $\displaystyle\sqrt{{9}}\usepackage{cmbright}\Huge\$ . However, this is not the case, because $\displaystyle-\sqrt{{-{9}}}=-\sqrt{{9}}\sqrt{{-{1}}}=-{3}{i}\usepackage{cmbright}\Huge\$ , which is purely imaginary.