Is the (x-a) format necessary when finding the radius of convergence for a geometric series?

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So I am taking AP Calculus BC, and we are currently working on convergence and divergence of series. I came across the following problem in one of my homework assignments:

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Here is the work I did to find the radius of convergence: enter image description here

My teacher said I shouldn't have added 5 to each side, and left it at the highlighted step, meaning 6 would be the radius of convergence. I asked him why, and he said that the -5 is what centers the function.

Why is this the case? Does it have to do with Taylor Polynomials at all? If I do add 5, then isn't the radius still 6 because (11--1)/2 = 6

If any of you have an intuitive explanation for this, I would really appreciate it!