I will only give you a hint for this problem because I think you have probably not submitted it entirely and/or correctly.
If f(x)=9/(x-6), write this as y=9/(x-6).
Interchange the roles of x and y:
x=9/(y-6)
solve for y
y=6+(9/x)
and f-1(x)=6+(9/x)
You will probably also want to know about derivatives...and I will leave that to you.