To determine the inverse of a function we can follow these steps:
1. Replace f(x) by "y".
2. Switch the places of "y" and "x" on the expression.
3. Isolate the "y" variable on the left side.
4. Replace "y" by f^-1(x).
Let's apply these steps to the given expression:

Now we can apply the second step:

We can now follow the third step:

Now the final step:

The expression for the inverse is (x+8)/9.