To find how much flour Makenna will have left, you will substitute (replace the letter c with the number of cakes she made. The answer you get will be how much flour is left.
f = –9c + 80
f = -9(3) + 80
f = -27 + 80
f = 53 cups
Makenna will have 53 cups of flour left after baking 3 cakes.