You may have correctly added $\displaystyle-{30}+{\left(-{15}\right)}\usepackage{cmbright}\Huge\$ to get $\displaystyle-{45}\usepackage{cmbright}\Huge\$ , but then incorrectly added $\displaystyle-{45}+{80}\usepackage{cmbright}\Huge\$ . When you are adding numbers with different signs, and the positive addend has a greater absolute value, the sum is positive. If the negative addend has a greater absolute value, the sum is negative.