You probably miscalculated $\displaystyle-\frac{{3}}{{7}}{\left({28}\right)}\usepackage{cmbright}\Huge\$ and got $\displaystyle-{84}\usepackage{cmbright}\Huge\$ , instead of $\displaystyle-\frac{{84}}{{7}}\usepackage{cmbright}\Huge\$ , to get the answer.