If you have 120 feet of fencing and want to enclose a rectangular area up against a long, straight wall, what is the largest area you can enclose?

The area of rectangular is  A= length x width
 
  120 feet of fancing = (the above line)

        !!                                     
        !                                                   !
     x !   the enclosed area                 ! x
        !                                                   !
        ! !
                          120 -(x+x)
so the area is  A = (120-2x) (x)= 120x-2x²