500
Mr. Stauffer's old yard has a perimeter of 18 feet. He already has fencing purchased to put around it. His new yard has a length of 6 and a width of 7. How much more fencing does he need to purchase to go all the way around the new yard?
Perimeter of new yard = 6+6+7+7 = 26 feet
New perimeter - old perimeter = 26-18 = 8 feet.
Mr. Stauffer needs 8 more feet of fencing.