A derivative is used to find the slope of a curve at any point along that curve. The function f'(a) is read as "f prime of a" and can be found using either definition of a limit, either as x approaches a or as h approaches 0.
Below are two examples showing one function being solved with both definitions. The key to solving these functions is to isolate a factor of the denominator in the numerator and cancel those factors out so the function is no longer dividing by zero. For x approaching a, isolate the term (x-a) in the numerator; or for h approaching 0, isolate the term h in the numerator.