You may have found the sum of $\displaystyle-{3}\frac{{1}}{{4}}\usepackage{cmbright}\Huge\$ and $\displaystyle\frac{{2}}{{3}}\usepackage{cmbright}\Huge\$ as your first step, rather than the sum of $\displaystyle-{3}\frac{{1}}{{4}}\usepackage{cmbright}\Huge\$ and $\displaystyle-\frac{{2}}{{3}}\usepackage{cmbright}\Huge\$ . To add the two negative numbers, you must add $\displaystyle{3}\frac{{1}}{{4}}+\frac{{2}}{{3}}\usepackage{cmbright}\Huge\$ and put a negative sign in front of that result. Then add $\displaystyle{9}\frac{{5}}{{8}}\usepackage{cmbright}\Huge\$ .