You probably did the first subtraction correctly to get $\displaystyle-{9.4}\usepackage{cmbright}\Huge\$ , but then got the wrong sign for $\displaystyle-{9.4}+{11}\usepackage{cmbright}\Huge\$ . Since $\displaystyle{11}>{9.4}\usepackage{cmbright}\Huge\$ , this should result in a positive 1.6. Then you probably subtracted 38.5 as your last step. You should be subtracting $\displaystyle-{38.5}\usepackage{cmbright}\Huge\$ , which is the same as adding 38.5.