Answer
See below
Work Step by Step
a) the average rate of change over an interval is simple the slope formula or
$$\triangle y/\triangle x = \frac{f(x_1)-f(x_2)}{x_1-x_2}$$
b) the instantaneous rate:
$$\lim\limits_{x \to 0}\triangle y/\triangle x = \lim\limits_{x_1 \to x_2}\frac{f(x_1)-f(x_2)}{x_1-x_2}$$
