Exercise 18 Page 16

To begin with, we need to figure out where the two planes are located in the diagram. Points $T,$ $X,$ and $W$ are all on the back face of the rectangular prism. Therefore, when we extend that face in all directions, we obtain plane $TXW.$ Similarly, plane $TQU$ is the left face of the prism extended in all directions.

These planes intersect at the left border of the back face (or the back border of the left face!). This occurs along the line which connects points $X$ and $T,$ $\overleftrightarrow{XT}.$