In a trapezoid, adjacent angles are supplementary, meaning they add up to 180°. ∠W and ∠WXY are adjacent, so they are supplementary. 180° - 73° = 107°.
Another way to do this is to know that there are 360° in a trapezoid. This is a right trapezoid, so two of the angles are 90°. One angle is 73°. 90° + 90° + 73° = 253°. 360° - 253° = 107°.
