Use the partial fractions method to express the function as a power series (centered at $x=0$) and give the open interval of convergence.

$$f(x) = \frac{9}{x^2 - 3 x-18}$$

$f(x) = \displaystyle\sum\limits_{n=0}^{\infty}$

The open interval of convergence is:
Give your answer in interval notation.