The concept of a derivative
This is an applet to help you understand the concept of a derivative. I am using the equation fro slope to calculate the slope of the secant line.
Questions you want to answer: What happens to the secant line the closer the 2 points get to each other? If you fixed one point at - 2 and moved the other point to closer and closer to that point, what does the slope seem to be approaching? How about -1, 0, 1, 2, 3?