Q:  Define a Price Floor.
A:  A minimum price at which goods or services are allowed to trade.

Q:  If Demand is P=75-Q and Supply is P=-25+Q, find Producer Surplus.
A:  First we must find the equilibrium price and quantity. 
        To do this set Supply and Demand equal.
        75-Q=-25+Q or 100=2Q or Qe=50. 
        Plug this Q back into Supply or Demand and you find that Pe=75-50=25.
        Now we need to see what shape PS has. 
        Since Supply crosses the quantity axis at a positive number, PS is not a triangle.
        To find this number plug in a price of 0 into the Supply curve.
        You should find the x-intercept is 25.
        So PS=25*25+(1/2)*(50-25)25= $937.50