You may have correctly added $\displaystyle{18}+{36}\usepackage{cmbright}\Huge\$ to get 54, and then incorrectly added $\displaystyle{54}+{\left(-{76}\right)}\usepackage{cmbright}\Huge\$ . To add two numbers with different signs, you must find the difference of their absolute values ( $\displaystyle{76}-{54}\usepackage{cmbright}\Huge\$ ). If 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.