I need help with the problem below (I'm not sure where to start...this was in the same section as rotating a region and finding volumes and integrations.....I'm not sure where the fits in) ....Thanks!

Here is the problem:

The circumference of a tree at different heights above the ground is given in the table below. Assume that all horizontal cross-sections of the tree are circles.

Height (inches) 0 30 60 90 120

Circumference (inches) 30 26 20 12 4

(a) What is an upper estimate of the volume of the tree from the ground to a height of 30 inches?

(b) What is an upper estimate of the total volume of the tree? Use N=4.