Determine free-market equilibrium price and quantity

Question

In the year of 1983, the Reagan Administration introduced a new agricultural program known as the Payment-in-Kind Program. To distinguish how the program worked, let's assume the wheat market. Assume the demand function is Q^D = 28 - 2P and the supply function is Q^S = 4 + 4P, where P refers to the price of wheat in dollars per bushel and Q refer to the quantity in billions of bushels. Determine the free-market equilibrium price and quantity.
Equating demand & supply, Q^D = Q^S,
                                                        28 - 2P = 4 + 4P, or P = 4.
To find out the equilibrium quantity, substitute P = 4 into either the supply equation or the demand equation:
                                                          Q^S = 4 + 4(4) = 20
                                                                     and
                                                           Q^D = 28 - 2(4) = 20.