Help with another math word problem!?

A large feed uses two feed supplements, Ration I and Ration II. Each pound of Ration I contains 4 units of protein and 2 units of carbohydrates. Each pound of Ration II contains 3 units of protein and 6 units of carbohydrates. If the dietary requirement calls for 42 units of carbohydrates, how many pounds of each ration should be used to satisfy the requirements?