2. For the tetrahedron $P = (2,0,0)$, $Q = (0,3,0)$, $R = (0,0,4)$, $S = (1,1,1)$, which faces are visible from above by an orthographic projection, i.e., from the direction k$= (0,0,1)$? Use a computer method that would be valid for any tetrahedron.