Angles YXZ and WXU are vertical angles.
These two angles are vertical angles because they are vertically across from each other. By definition, vertical angles are a pair of angles formed by two intersecting lines. Also, remember that vertical angles are congruent, according to the Vertical Angles Theorem.
Remember, when referring to an angle, you must always list a point on one of the rays, then the point of intersection of two rays, then the point on the other ray. So, the intersection of the two rays will always be the middle letter. For example, in angle WXU, W is a point on the ray, X is the point of intersection of the two rays to form the angle, and U is a point on the other ray.