We know that any electron which passed through the slit
  must have had a y-position which was between the bottom
  and top of the slit: a distance d.

  Thus, the uncertainty in the y-position of such electrons 
  is

            delta_y   =    d


  Using Heisenberg's Uncertainty Principle, we can say


                                    h
       (delta_y) * (delta_py)  ~   ----
                                   2 pi



                                    h
           d     * (delta_py)  ~   ----
                                   2 pi


                                     h
                   (delta_py)  ~   ------
                                   2 pi d