A farmer can buy two types of plant food, mix A and mix B. Each cubic yard of mix A contains 36 pounds of phosphoric acid, 36 pounds of nitrogen, and 6 pounds of potash. Each cubic yard of mix B contains 12 pounds of phosphoric acid, 30 pounds of nitrogen, and 27 pounds of potash. The minimum monthly requirements are 540 pounds of phosphoric acid, 900 pounds of nitrogen, and 270 pounds of potash. Find the set of feasible solutions graphically for the amounts of mix A and mix B that can be used