You may have mistakenly believed that $\displaystyle\angle{P}{U}{R}\usepackage{cmbright}\Huge\$ measured $\displaystyle{{90}}^{\circ}\usepackage{cmbright}\Huge\$ . You need to use the fact that, because $\displaystyle\angle{P}{U}{T}\usepackage{cmbright}\Huge\$ is a straight angle, it measures $\displaystyle{{180}}^{\circ}\usepackage{cmbright}\Huge\$ . The four angles along this straight angle have measures that total $\displaystyle{{180}}^{\circ}\usepackage{cmbright}\Huge\$ . Use the three given measures to find the fourth.