(a) If
f ( 8 )

Since $f$ is a one-to-one function, the inverse function $f^{-1}$ undoes the action of $f$. That is, if $f(8) = 7$, then the inverse $f^{-1}(7) = 8$.

Thus, the answer is: $f^{-1}(7) = 8$

Would you like a detailed explanation on inverse functions, or do you have any other questions?

Here are 5 related questions you might find helpful:

1. What does it mean for a function to be one-to-one?
2. How do you verify if a function has an inverse?
3. What is the graphical interpretation of inverse functions?
4. Can a function have more than one inverse?
5. How do you find the inverse of a quadratic function?

Tip: A function is one-to-one if and only if every horizontal line intersects its graph at most once (this is called the Horizontal Line Test).