# How to compute the 3D location of a 2D point on the ground?

I know the focal length f and the principal point of a camera P(x,y) and I can assume the ground plane is orthogonal to the image plane.

I have a 2D point on the picture that I know is on the ground, how do I get it on 3D using camera coordinates. However I am not sure how to approach this? Any advice is appreciated.