how to calculate angle collision

Hi everybody.

First post but straight to the point:

I know the coordinates of point A, and draw a line untill it hits point B.
How do i recalculate so that the line is drawn to point C? All i need is the coordinates of point C in 3d space or the angle from B to C.

Unfortunatly my maths are pretty bad and I just cant seem to get it.
Any help is greatly appreciated

(example of what i'm after)