My brain's frazzled, can anyone help? For some reason I can't think of the most efficient way to do the following:

Imagine the world is a flat plane at Y=0. An object A is somewhere on this plane, facing in any direction along the plane. So its position is (anything,0,anything) and euler angles are (0,anything,0) Object B is somewhere else on the plane, direction irrelevant.

I want to work out the relative Y-axis angle from A to B. Assume I don't have access to transforms, only their positions and A's euler angles. So the answer should be a float giving what should be added to A's euler angle Y-axis in order to face B.