C# XNA Mouse position projected to 3D plane
===

I'm working on a 3D XNA project, and I've been thinking about this problem for like 2 weeks.
So I just decided to ask you.

Basically I have a flat plane and i want to project the mouse position to that plane, but how?
I tried many ways to do it, calculated angles... 
But i figured out, that the distance must effect on the X position, maybe some math is needed what I've never heard before.

Thanks for your time.
