Urgent, please help

Question 5: [20 pts] Continuing with the previous question, instead assume that an infinite period game in which firms agree to collude by setting the monopoly price each period every period forever. Assume the discount factor equals to r. (a) Suppose the trigger strategy is "In each period, set price equal to the monopoly price, provided each firm has set price equal to the monopoly price in every past period since the agreement began. If any firm did not set the monopoly price as required in any past period, then set price equal to marginal cost (i.e. now and forever after)." How high does the discount factor have to be to make the cartel stable? (b) How does your answer to (a) change if it takes two periods for firms to detect "cheating" instead of one