I'm assuming there is supposed to be a x next to the -9. If that is the case, the inverse is:
f(x) = (x - 8)/-9
You can find this by switching the x and f(x) values and then solving for the new f(x) value.
x=-9f(x)+8 ---> subtract 8 from both sides
x - 8 = 9f(x) ----> divide both by -9
(x - 8)/-9 = f(x)
