solve this problem im stuck in the question

QUESTION 1 (10 marks) The urn U1 contains 5 red and 4 green balls and the urn U2 contains 4 red and 7 green balls. The urns are identical, the balls are not visible from outside, and it is not known which urn is U1 and which is U2. Consider the following experiment: One urn is chosen at random. From this urn, a ball drawn and returned to the same urn. If the ball is red, a next ball drawn from the same urn and returned again. This procedure is repeated until a green ball is drawn and returned. Thereafter, one last ball is taken from the same urn. Suppose that three red balls have been drawn before a green ball appeared. Calculate the probability that the last ball is red