Exercise 34 Page 385

To begin with, we have to figure out where the two planes are located in the diagram. The points $B,$ $G,$ and $F$ are all on the back face of the cube, so when we extend that face in all directions, we obtain plane $BGF.$ Similarly, plane $HDG$ is the right face of the cube extended in all directions.

These planes intersect at the back border of the right face. This occurs along the line which contains point $C$ and $G,$ $\overleftrightarrow{CG}.$