You may have chosen this option because it contains $\displaystyle\frac{{7}}{{2}}\usepackage{cmbright}\Huge\$ and i, but it is not written as a complex number. A complex number is any number in the form $\displaystyle{a}+{b}{i}\usepackage{cmbright}\Huge\$ where a and b are real numbers. The fraction $\displaystyle\frac{{7}}{{2}}{i}\usepackage{cmbright}\Huge\$ is only the imaginary part of this complex number; you also need to write the real part for this to be a complex number.