A fair coin (H for heads and T for tail, that are equally likely to occur) is tossed UNTIL a head appears once or a total of three tails have appeared. Flips of the coin are independent

Let X represent the number of tails that occur in the experiment. Determine the probability mass function of X.

Let Y represent the number of heads that occur in the experiment. Determine the probability mass function of Y.

Confused on how to set up the formula since this is a Negative Binomial Distribution.