If that's so, then the following may be helpful.
Suppose that has coordinates and that the equation of the line is . Suppose further that is the projection of onto , and that the coordinates of are .
Then, because lies on :
and because :
Substitute from (1) into (2):
Substitute into (1):
So equations (3) and (4) give us the coordinates of Q in terms of and .
Of course, you may be able to simplify these results if you are able to choose a particular origin of coordinates. For example, if the origin can be chosen to lie on the line , then , and equations (3) and (4) simplify to:
Or, if you could also choose two lines at to the axes, then you can make , and the equations would simplify even further to:
I hope this gives you some ideas.