A tree typically lives 20 years and

Given the dimension of a tropical tree, calculate how much carbon they contain when they are alive.
There are an average of 200 trees per acre in the rainforest.
Calculate the amount of carbon released when 1 acre of rain forest is cut down or burned.

Can someone explain how to do this?

http://www.unm.edu/~jbrink/365/Documents/Calculating_tree_carbon.pdf