Exercise 17 Page 16

To begin with, we need to figure out where the two planes are located in the diagram. Points X, W, and V are all on the top face of the rectangular prism. Therefore, when we extend that face in all directions, we obtain plane XWV. Similarly, plane UVR is the front face of the prism extended in all directions.

These planes intersect at the upper border of the front face (or the front border of the top face!). This occurs along the line which connects points U and V, UV.