The part that is confusing is the "per acre" expression. If that factor was supposed to be included, then the number of acres will be needed to solve the problem.
Otherwise, it appears that 3 pounds of pesticide is used in 400 gallons.
If that analysis is correct, then 3 pounds ÷ 400 gallons = 0.0075 pounds per gallon
So, we then multiply 0.0075 pounds per gallon by 15 gallons = 0.1125 pounds.