to get the marginal of T, you need to sum over x.
Suppose that the random variables X and T have the joint probability mass function
p(x,t) = P(X=x, T=t) = p^2(1 − p)^t-2, t = x+1, x+2,···, x = 1,···,t − 1,
where 0 < p < 1.
Find the marginal probability mass function of T.
I really have no clue how to do this question