Be aware that although the Dell printers are re-badged Lexmark ones, they have deliberately changed the mechanical interface on the cartridge to try to stop you using much cheaper Lexmark, or Lexmark compatible, cartridges (guess how I found that one out......).
However, do not despair! With a sharp knife you can remove the extra plastic bulge on the top of the Lexmark cartridge to make it fit the Dell version of the printer! Just compare the two cartridges to see which is the bit to remove.
VP