Exercise:

For which values of a ∈ R are the points P = (-1, 1, 2), Q = (0, a, 1), R = (a, 4, -1) and S = (-11, -1, 0) corners of a tetrahedron? Cacluate the volume of the tetrahedron PQRS for these values of a.

I really need some advice on how to execute this on, as I have no idea where to begin.

Best regards.