Please help me to solve these maths problems?

Please could you help me solve these two problems. I really can't do them. I just don't need the answer, but please I also need to know how to solve them.

Here:

As the moon orbits Earth the distance between them varies between 4.07 * 10^5 km and 3.56 * 10^5. Find the difference between these two distances.

The radius of Earth is approximately 6380 km. Find the volume of Earth in cm^3 giving your answer in standard forum correct to 3 significant figures. (use the formula V = 4/3 pii r^2)

Many thanks :)