You may have added the first two numbers and got $\displaystyle-{29}\usepackage{cmbright}\Huge\$ , but then subtracted 12 incorrectly. You should rewrite $\displaystyle-{29}-{12}=-{29}+{\left(-{12}\right)}\usepackage{cmbright}\Huge\$ . You need to add $\displaystyle{29}+{12}\usepackage{cmbright}\Huge\$ , and figure out the right sign. For the last step you need to remember that subtracting a negative number is the same as adding a positive number.