You may have done the first subtraction correctly to get $\displaystyle-{7.1}\usepackage{cmbright}\Huge\$ , but then got the sign of $\displaystyle-{7.1}+{9}\usepackage{cmbright}\Huge\$ incorrect. Since $\displaystyle{9}>{7.1}\usepackage{cmbright}\Huge\$ , this should result in a positive 1.9.