Rodney needs to make 3 pizzas and each pizza requires 2 cups of flour. He buys a bag of 20 cups of flour...

3 X 2 = 6

He makes 3 pizzas, so he uses 6 cups of flours total.

To find how much flour he will have left over, subtract 6 from 20. That will give you 14 cups of flour left over.