If the midpoint of the segment is at (12,-6) and one of the endpoints is at the point (-6, 18), what is the coordinate of the other endpoint?

There are three bags: a (contains 2 white and 4 red balls), b (8 white, 4 red) and c (1 white 3 red). you select one ball at random from each bag, observe that exactly two are white, but forget which ball came from which bag. what is the probability that you selected a white ball from bag a?