Since you know the formula, you are very close to it, and I don't want to spoil the fun. I will give the hint.
Your x's are already been stated in the question. First know what exactly is in you sample space, S. Knowing your x's and S, you should know your P(X=x).
If you know part (a), you will know part (b) by experimenting n=1, 2, 3.
From n=1 to 3, you should see the pattern, then express it using ∑ for the range from 1 to n.