So I have 4 points, one of them has an unknown x co-ordinate. How would I show that they are coplanar? With 3 points I could just turn them into vectors, work out the cross product and conclude they lie in the same plane if the answer is 0, right? But the 4th point and the unknown is throwing me. Any hints? Edit: Well, I need to eventually work out what value of the unknown x would satisfy, um, co-planarity (you know what I mean).