(sorry, low level so I can't attach the image in the post)
- a is given.
- b is given.
- c is given.
- For X, I don't have a clue where he is.
I was thinking about rect / vector instersection, projection, and other things but still I couldn't come with a solution. If it's computer light, the better.
Sorry for not making clear before: ab and bx are not always orthogonal.