Okay, so this question gives you 7.5 cups of flour for 3 dozen cookies. They want you to figure out how much is needed for 15 dozen, so divide 15 dozen cookies by 3 dozen cookies, and you get 5. This means that you will need the amount of flour that you need for 3 dozen cookies, 5 times.
So, multiply 5 by 7.5 cups of flour, and you will have your answer. (: