There are several important information's already given in the question. Based on those information's the answer to the question can be easily deduced.
Number of cups of Oatmeal
required by Aunt Bess for making 6 dozen cookies = 3
Then
Number of cups of oatmeal
required by Aunt Bess for making 1 dozen cookies = (3/6)
= 1/2
So
Number of cups of oatmeal
required by Aunt Bess for making 18 dozen cookies = (1/2) * 18
= 9 cups
So Aunt Bess will require 9 cups of oatmeal for baking 18 dozen cookies. I hope the procedure is clear enough for you to undestand.