You may have correctly added $\displaystyle-{3}\frac{{1}}{{4}}\usepackage{cmbright}\Huge\$ and $\displaystyle-\frac{{2}}{{3}}\usepackage{cmbright}\Huge\$ to get $\displaystyle-{3}\frac{{11}}{{12}}\usepackage{cmbright}\Huge\$ . When you combined this with $\displaystyle{9}\frac{{5}}{{8}}\usepackage{cmbright}\Huge\$ you gave your answer the incorrect sign. Since $\displaystyle{9}\frac{{5}}{{8}}>{3}\frac{{11}}{{12}}\usepackage{cmbright}\Huge\$ , the answer is positive.