what does the second derivative of the geometric moment generating function yield? is it 1/p^3
There is a difference between being stuck because you just don't know what to do, and being stuck because what you have to do is tedious and requires great care. In all honesty I think you're 'stuck' falls into the latter category. You just have to apply the quotient rule and put in all the steps. I think you can do that task just as well as I can - probably better since you will be motivated.
If you genuinely cannot do it, please say so. Preferably, show where abouts in the calculation you were able to get to.