Can u guys pls help me on this question and pls explain how u got the answer

Place the following steps in order to complete the square and solve the quadratic equation,

1yd 1 ft = 1 1/12 1 1/3 1 1/2 1 1/6

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?