If we have equations for both supply and demand we can solve for the equilibrium price and the equilibrium quantity by setting Q_Sand Q_D equal to one another, and solving for the price; which equates the desired quantity supplied to the desired quantity demanded. For this case we have:

         Q_S = Q_D
   -20 + 2P = 220 - 4P
         6P = 240 
         P* = 40  
         Q* = 60

For this market, the equilibrium price is 40 and the equilibrium quantity is 60.