You may have correctly recognized the perfect square 100 as a factor of 500, and correctly used $\displaystyle\sqrt{{-{1}}}={i}\usepackage{cmbright}\Huge\$ , but you forgot the remaining factor of $\displaystyle-{500}\usepackage{cmbright}\Huge\$ , which is 5. Use the rule $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{{a}}}\sqrt{{{b}}}\usepackage{cmbright}\Huge\$ and make sure that you write down every step. The correct answer is $\displaystyle{10}{i}\sqrt{{{5}}}\usepackage{cmbright}\Huge\$ .