You added the absolute values of the temperatures instead of subtracting $\displaystyle-{36.1}-{\left(-{47.3}\right)}=-{36.1}+{47.3}\usepackage{cmbright}\Huge\$ to find the difference in temperatures. Also, though $\displaystyle{{47.3}}^{\circ}\usepackage{cmbright}\Huge\$ is warmer than $\displaystyle{{36.1}}^{\circ}\usepackage{cmbright}\Huge\$ , $\displaystyle-{{36.1}}^{\circ}\usepackage{cmbright}\Huge\$ is warmer than $\displaystyle-{{47.3}}^{\circ}\usepackage{cmbright}\Huge\$ .