The probability of an event is the number of ways that event can occur divided by the total number of possible events. In your case, it is the number of red marbles divided by the total number of marbles.
For the first marble, the probability of red is 9/(9+2) = 9/11.
For the second marble, the probabilities are a little difference because the first marble was drawn without being replaced. Since the first marble red, there is now one less red marble that can be chosen on the second draw. So the probability is now: 8/10.
Hope that helps.