You may have chosen this number because it has the right form for a complex number, but you turned a product (of 9 and i) into a sum. A complex number is any number in the form $\displaystyle{a}+{b}{i}\usepackage{cmbright}\Huge\$ where a and b are real numbers. The correct answer is $\displaystyle{0}+{9}{i}\usepackage{cmbright}\Huge\$ .