Answer :
Well first of all the kilogram is a unit of mass not a unit of weight. Your teacher should never have asked for weight in kilograms. If your class is meeting on Earth then one kilogram of mass weighs about 2.2 pounds. You can just divide your 8.4 pounds by 2.2. The quotient is 3.9 and that's the mass of the milk in kilograms. Be careful. If your class is meeting on some other planet then one kilogram of mass doesn't weigh 2.2 pounds there.