I am assuming that you are talking about the angle wyx.

At the same time you are saying yx is parallel to wx. That means that the line segment yx is right on top of the line segment wx as x is the common point to both line segments.

This means that either the point w lies between points y and x. Or the point y lies between points w and x.

If point w lies between y and x, then the angle wyx = 0 degrees.

If point y lies between w and x, the the angle wyx = 180 degrees.