There are 9+7+6=22 marbles in total. The probability of selecting a yellow marble would be 9/22. By replacing that yellow marble with a green marble, you aren't changing the total marble count, so the probability of selecting a green marble would be 7/22. Because the two events are independent, their probabilities will get multiplied. Therefore, P(Yellow, then Green) = P(Yellow) * P(Green) = 9/22 * 7/22 = 63/484.
