How do I solve this question? This is the entire exact question given in the assignment.

6. You learn that you have won the lottery. In your excitement you open your piggybank which has 36 coins in it, and you jubilantly throw them all up in the air. They randomly land on the ground. (a) Assuming all the coins are fair, what is the probability that exactly half of the coins are tails? (b) It turns out that 2 of the 36 coins are not fair. For one coin, P(Heads) = 0.7 and P(Tails) = 0.3. For the other coin, P(Heads) = 0.9 and P(Tails) = 0.1. The renaming coins are all fair. In this case, what would be the probability that after the throw exactly half of the coins are tails?