Hi! :welcome: We are glad that you posted here! This looks like a homework question though. Our homework help policy can be found here. We mainly just want to see what you have tried so far and that you have put some effort into the problem. I would also suggest checking out this thread for some guidelines on smart posting behavior that can help you get answers that are better much more quickly.

My first thought is to find P(X >=4) for f(x) and P(X >= 1) for f(x). Then I take the results and compute the conditional probability for f(x) P(X >= 4 | X >= 1).

So, P(X >= 4) is equivalent to the SUM (with k=4 to infinity) f(x)^k - f(x=0) - f(x=1)- f(x=2) - f(x=3).
Similarly, (P >= 1) = SUM (w/ k=1 to infinity) f(x)^k - f(x=0).