You may have correctly noticed the perfect square 64 as a factor of 128, and correctly used $\displaystyle\sqrt{{-{1}}}={i}\usepackage{cmbright}\Huge\$ , but you forgot the remaining factor of $\displaystyle-{128}\usepackage{cmbright}\Huge\$ , which is 2. Use the rule $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{{a}}}\sqrt{{{b}}}\usepackage{cmbright}\Huge\$ and make sure that you write down every step.