Ok thanks. I see what I did wrong now. Also:

Basically I have no idea what this is trying to tell me. This is from the online lecture notes my lecturer put up. I have no idea what is going on, and no idea what h is.

1. Can someone please explain what is going on in steps, really simply.

2. what is h?

I know the basic idea is to make the two points closer and closer together, and then get the gradient of those two points when they are almost identical... But as you can see I don't get how to do it and what this is telling me.