The freezing point for Substance B is found by adding $\displaystyle-{185}+{126}\usepackage{cmbright}\Huge\$ . You added the absolute values of the addends, and applied a negative sign. Since the addends have different signs, so you must find the difference of the absolute values and give the sum the same sign as the addend with the greater absolute value. The correct answer is $\displaystyle-{{59}}^{\circ}\text{C}\usepackage{cmbright}\Huge\$ .