In the future, it helps those trying to answer your question if you post your question in the proper place. This is pretty clearly not a problem from an elementary school classroom. You see, the tough thing is that I don't know what your class material looks like so I'm not sure what way I ought to help (there is more than one way to do this problem).

That said, I'll just make assumptions and say that your fourth point D will be to point A what point C is to point B. What I mean by that is, imagine the vector from B to C (not hard, it's the side of the parallelogram between B and C). Now take that vector and add it to A to find D. You do the working out--draw a picture if you have a hard time picturing it and it'll pop right out at you.