You subtracted $\displaystyle{47.3}\usepackage{cmbright}\Huge\$ from $\displaystyle{36.8}\usepackage{cmbright}\Huge\$ instead of rewriting the problem as $\displaystyle{36.8}-{\left(-{47.3}\right)}={36.8}+{47.3}\usepackage{cmbright}\Huge\$ . The correct answer is 84.1.