So, to make sure I'm following you, Mark44, I would end up with a function r of s and t. OP= <1,1,-1,2>, u=PQ=<1,2,1,-1> , and v=PR=<0,1,3,1>. r(s, t) = OP + su + tv
r(s, t) = <1,1,-1,2> + s<1,2,1,-1> + t<0,1,3,1>
I know I need to have a point in the plane and two vectors from that point...