You may have added $\displaystyle{23}\frac{{7}}{{8}}+{10}\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ , and then supplied the answer with a negative sign. However, you need to find the difference of $\displaystyle{23}\frac{{7}}{{8}}-{10}\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ . The answer will be negative, because $\displaystyle{23}\frac{{7}}{{8}}>{10}\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ .