I think the following picture helps explain it easier than words:
I know the coordinates of point a, which is at a corner of the triangle, as well as the coordinates of point b, which is somewhere along the edge of the opposite side. I also know the y-coordinate of point c, and I'm trying to find its x-coordinate.
What I've got so far is that as the y-coordinate of b moves up to c, the x coordinate of c approaches the x coordinate of b. And as the y-coordinate of b moves down to a, the x coordinate of c moves to half-way between the x coordinates of a and b. But I don't know the direct relationship.