Find the vertical asymptote(s) of the function, if any.
$f(x) = \frac{x-9}{x-3}$
Complete the equation of the vertical asymptote. If there are multiple vertical asymptotes, separate the answers with commas (,). For example, if there are vertical asymptotes at $x = 2$ and $x = 3$, then enter "2,3". If there are no vertical asymptotes, enter
"none".