Again, what I said in the other post.

If you have read that post first, (2/3) * sum (n=0~inf.) (x/3)^n should make sense to you.

Now, time for some algebraic manipulation: anything other than n can enter or exit the sigma sign FREELY.

(2/3) * sum (n=0~inf.) (x/3)^n = (2/3) * sum (n=0~inf.) x^n/3^n = 2 * sum (n=0~inf.) x^n/3^n * (1/3) = 2* sum (n=0~inf.) x^n/3^(n+1).