Question: how to compute the difference quotient?
Answer: Plug x + h into the function f and simplify to find f(x + h).
Now that you have f(x + h), find f(x + h) - f(x) by plugging in f(x + h) and f(x) and simplifying.
Plug your result from step 2 in for the numerator in the difference quotient and simplify.