You may have correctly found that $\displaystyle-{24}+{15}=-{9}\usepackage{cmbright}\Huge\$ , but then subtracted $\displaystyle-{9}-{29}\usepackage{cmbright}\Huge\$ incorrectly. To do this correctly, first note that $\displaystyle-{9}-{29}=-{9}+{\left(-{29}\right)}\usepackage{cmbright}\Huge\$ . Then you should add $\displaystyle{9}+{29}\usepackage{cmbright}\Huge\$ and make the result negative.