Thread: Convergence: The domain and radius

I did the following calculations.
I need to find the domain and radius.
I think the radius is (-R,R) AKA (-2,2).
What is the domain ? I think it should be in the form of (a-R,a+R). If, how do I determine "a" ?
What does imply that it is the convergent series ?

This has nothing to do with number theory post in correct subforum.