The preceding should read "Given three points, P1, P2, and P3, which, together, define a plane, P, a triangle, and three lines, L1, L2, and L3, such that at least two of the lines are not parallel to and therefore do intersect the x - y axial plane, define a line formed by the intersection of P and the x - y plane."
This is part of another problem: given three points that (as they must do) define a plane, define the plane. We suspect that defining the plane means deriving an equation which gives the value of z for any point on the plane given the values for x and y for that point.
This in turn is part of yet another problem: given a defined plane and a defined line, find the intersection of the line and the plane. The intersection of a line and a plane is a point, and what we seek to calculate are that point's x, y, and z coordinates.
