You may have correctly added $\displaystyle{18}+{36}\usepackage{cmbright}\Huge\$ to get 54, but then incorrectly added $\displaystyle{54}+{\left(-{76}\right)}\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.