You added the opposites of $\displaystyle-{52.8}\usepackage{cmbright}\Huge\$ and $\displaystyle-{68.9}\usepackage{cmbright}\Huge\$ . You should just add the opposite of $\displaystyle-{68.9}\usepackage{cmbright}\Huge\$ , so the problem becomes $\displaystyle-{52.8}+{68.9}\usepackage{cmbright}\Huge\$ . The correct answer is 16.1.