Representations of functions as power series

Question

Find a power series representation for the function.

f(x) = (8+x)/ (1-x)²

^{f(x) = ∑(from n=0 to ∞) ( ? )}

I got the answer ∑ (from n=0 to ∞) (9n-1)x^n

but the professor marked my answer wrong! What might be wrong with my answer?

Answer 1

Use 9n+8 instead of 9n-1, if n starts at 0.