Economic question

Hi sir, I have a very poor economic professor; I am having trouble with this question. I want to know how I can get the answer to this, calculate the marginal cost, and calculate an optimal choice. There are 60 questions I have to complete, but the textbook doesn't address the same concepts as the assignment itself.

Suppose the total benefit and total cost to society of various levels of pollution abatement are as follows:
Complete column (4). (Enter your responses as integers.)
Pollution
Abatement
Pollution
Abatement
Total Benefit
Total Benefit
Total Cost
20
33
48
65
Total Cost
20
33
48
65
Total Net
Benefit
11
21
20
15
Total Net
Benefit
11
18
20
15
Marginal
Benefit
Marginal
Benefit
Marginal
Cost
Marginal
Cost
using optimization in levels, if the U.S. Environmental Protection Agency (EPA) wants to maximize total net benefit, then
unit(s) of pollution abatement. (Enter your response as an integer.)
it should require