Hey, I'm having trouble understanding the process to find the expectation of pmf's that I have to find.

I know the formula for it but still cannot understand it.

With The first link, I can get all the way to question (c)

I get the pmf, $\displaystyle P(X=x) = 1/4*(2/3)^{k-2} for (K=1,2,3,.....)$

But can't get the expectation, and for the second link i'm completely clueless