[Danr55]

If A B C is a triangle, you don't have to move B at all. It is already in line with C. If A B C is a straight line, you essentially create a triangle when you move A so you would solve for the height of the triangle which would be the distance from point B to line AC. That would depend on the distance from A to C. Really need more information regarding the preliminary relationship between A B and C. If they are a straight line, it would help to know either the distance AC or BC to have a real solution. It almost sounds like we are adjusting iron sights..