It is given that a box contains 3 black and 4 white marbles where each trial is independent of the other
Sample space S = {1B, 2B, 3B, 1W, 2W, 3W, 4W}
Consider success marble drawn is black = 3/7
So the probability of success in the first trial
P1 (success) = 3/7
Consider probability of success in the second trial without replacement of the first draw
P2 (success) = 2/6
So the probability that both the marbles drawn are black, with each trial being independent