Fix LaTex
The first thing you do is take a Calculus course or at least check a Calculus text out of a library!
You will find that, for a function, f(x), the "difference quotient", that Jomo refers to, at x= a is defined as \(\displaystyle \frac{f(x+h)- f(a)}{h}\). The derivative of f(x) at x= a is the limit, as h goes to 0 of that difference quotient. Do you know anything about "limits"?