The definition of a derivative gives:
Now, use a little algebra to simplify and note that h heads toward 0 and you should be able to answer everything pertaining to that problem.
Before I try to solve it from what you told me can you please spot what my mistake is on my current work first?
Edit: Actually ignore the first attachment. (I changed the file so it's not here anymore anyways). I still have a problem though just not the same kind.