Newbie to Derivative needs help

There are 2 problems I am having particular trouble with.

QUESTION 1

A coal-burning electrical generating plant emits sulfur dioxide into the surrounding air. The concentration, C(x), in parts per million, is given approximately by C(x)= 0.1/x^2 where x is the distance from the plant in miles.

So the derivative is -.2x^-3 i believe?

And then I have to evaluate C(2) and C'(2).

C(2) = -.2(2)^3 = -.025

C'(2) = I am unsure of how to acquire this.

QUESTION 2

Use the definition of the derivative to find f'(x) if f(x) = x^3.

I know the derivative is 3x^2 but when I try to do it step by step by hand I get screwed up somewhere.

Any help or tips in the right direction would be appreciated.