Exercise 41 Page 385

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

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