Three men and three women line up at a checkout counter in a store.

a. In how many ways can they line up?

b. In how many ways can they line up if the first person in line is a woman, and then the line alternates by gender- that is a woman, a man, a woman, a man and so on?

c. Find the probability that the first person in line is a woman and the line alternates by gender.

***I'm really stuck on this one, and help would be appreciated!!!!****