We use the formula P(X = x) = p(1 - p)^(x - 1) when we define "success" as the event we are interested in, and we want to know the probability that the first failure occurs on the x-th trial. In this formula, p is the probability of success, 1 - p is the probability of failure, and x represents the number of trials (Illowsky, et al.,2022)