3. (20 pt.) A standard deck of playing cards consists of 52 cards. Each card has a rank and a suit.

There are 13 possible ranks (2, 3, 4, 5, 6, 7, 8, 9, 10, J, Q, K, A), 4 possible suits (spades, clubs, hearts, diamonds), and 13 cards for each suit (one for each rank). How many different hands of 7 cards

a. (10 pt.) contain 4 cards of one rank and 3 cards of a second rank?

b. (10 pt.) contain 3 pairs of 3 different ranks and a single card of a fourth rank?