Start with $\displaystyle{17.3}-{24}={17.3}+{\left(-{24}\right)}\usepackage{cmbright}\Huge\$ . Since $\displaystyle{24}-{17.3}={6.7}\usepackage{cmbright}\Huge\$ , and $\displaystyle{24}>{17.3}\usepackage{cmbright}\Huge\$ , $\displaystyle{17.3}+{\left(-{24}\right)}=-{6.7}\usepackage{cmbright}\Huge\$ . Then add 11.9 to get 5.2. Finally, rewrite $\displaystyle{5.2}-{\left(-{16.4}\right)}\usepackage{cmbright}\Huge\$ as $\displaystyle{5.2}+{16.4}\usepackage{cmbright}\Huge\$ to get 21.6.