Exercise 40 Page 9

The points $P,$ $Q,$ and $N$ are all on the front face of the cube. This means that the front face is contained within plane $PQN.$ Therefore, the plane $PQN$ is as if the front face of the cube was extended indefinitely in all directions.

The points that are not coplanar with $P,$ $Q,$ and $N$ are then any points that are not on the front face of the cube. These are $K,$ $L,$ $R,$ and $S.$