Exercise 7 Page 390

The Segment Addition Postulate can be used to compute the length between two points if there is a third point between the points. The three points must be collinear, which means they have to be on the same line.

As we can see in the picture, the point $Y$ is not between $W$ and $Z.$ Therefore, we cannot use the Segment Addition Postulate to find the distance between $W$ and $Z.$