Exercise 15 Page 16

To begin with, we need to figure out where the two planes are located in the diagram. Points $Q,$ $R,$ and $S$ are all on the bottom face of the rectangular prism. Therefore, when we extend that face in all directions, we obtain plane $QRS.$ Similarly, plane $RSW$ is the right face of the prism extended in all directions.

These planes intersect at the right border of the bottom face (or the bottom border of the right face!). This occurs along the line which connects points $R$ and $S,$ $\overleftrightarrow{RS}.$