@zorodius no i mean why isnt the denominator, like in this example h, put in f(s)
Why would it be?
This particular problem is the way it is because you are being asked to work with the difference quotient, which is something that has a very important relationship to calculus. You're looking at what happens when you evaluate a function at two points a small amount apart, take the difference of those two evaluations (the "rise"), and divide it by the small amount they were apart (the "run"). In limit, this becomes the derivative, which you'll hear a lot more about in calculus.
They could ask you to evaluate functions and make substitutions without having anything to do with the difference quotient, though.