You did not find a common denominator for the fractions and incorrectly attempted to add and subtract the numerators and denominators. Add $\displaystyle-{2}\frac{{1}}{{3}}\usepackage{cmbright}\Huge\$ and $\displaystyle-\frac{{5}}{{6}}\usepackage{cmbright}\Huge\$ , using a common denominator of 6. When you combine this with $\displaystyle{7}\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ , use a common denominator of 12.