One way to subtract is by regrouping: $\displaystyle{9}\frac{{1}}{{2}}-{6}\frac{{3}}{{4}}={8}\frac{{3}}{{2}}-{6}\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ . Then find a common denominator: $\displaystyle{8}\frac{{6}}{{4}}-{6}\frac{{3}}{{4}}={2}\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ .