b) is a binomial, the expected number is np, where n=800 and p is obtained in (a)
For (a), there is other information that is missing, such as other people's top cards and how many players.
BUT if you just want to know the chance of a pairs it would be 3/51.
The first card can be anything and the chance you have a match on the second would be 3/51=1/17.
OR use the hypergeometric....