Need help with the following Probability problem:

Problem 6. (Bonus) (20 points) a. (5 points) From a set of 71 people, a committee of size 3' is to be chosen, and from this committee, a subcommittee of size i,z' S j is also to be chosen. Derive a combinatorial identity by computing, in two ways, the number of possible choices of the committee and subcommitte%rst by supposing that the committee is chosen rst and then the subcom mittee is chosen, and second by supposing that the subcommittee is chosen rst and then the remaining members of the committee are chosen. b. (5 points) Show that, for n > 0, Hint: Use the binomial theorem c. (10 points) Use part (a) and (b) to show that i(7;)(g)(1)\"j:0, i<>