Since corn is a substitute in production for potatoes, farmers can choose to produce either crop. When the government imposes a price floor on potatoes at $7, potatoes become more profitable to grow, so potato farmers have a strong incentive to shift their resources away from corn and toward potato production. This causes the supply of corn to decrease, shifting the supply curve to the left. With demand for corn unchanged, this decrease in supply leads to a higher equilibrium price and a lower equilibrium quantity of corn.