You may have added the numerators $\displaystyle{\left({3}+{4}\right)}\usepackage{cmbright}\Huge\$ and added the denominators $\displaystyle{\left({4}+{5}\right)}\usepackage{cmbright}\Huge\$ instead of multiplying. The correct answer is $\displaystyle\frac{{3}}{{5}}\usepackage{cmbright}\Huge\$ .