$\displaystyle\angle{V}{O}{X}\usepackage{cmbright}\Huge\$ is marked as a right angle. Therefore, the measures of $\displaystyle\angle{V}{O}{W}\usepackage{cmbright}\Huge\$ and $\displaystyle\angle{W}{O}{X}\usepackage{cmbright}\Huge\$ add up to $\displaystyle{{90}}^{\circ}\usepackage{cmbright}\Huge\$ . That makes these two angles complementary.