The commutative property says that when numbers are being multiplied they can be multiplied in any order and you will get the same result which is what that question shows by flipping the numbers around
I hope this helps and please don't hesitate to ask if there is anything still unclear!
If you need 3/4 cups of milk to make 8 waffles, you would need 9/4 cups to make 24 waffles because you are making 3 times the amount of waffles, so you need 3 times the amount of milk.
