Probability : Coin Flips

Independent flips of a coin that lands on heads with probability p are made.  What is the probability that the first four outcomes are:

(a) H, H, H, H

(b) T, H, H,H

(c) What is the probability that the pattern T, H, H, H occurs before the pattern
H, H, H, H?  JUSTIFY YOUR ANSWER BY DEFINING EVENTS