I just moved ibto a mobil home and the washer drain pipe is caped off at abot 18in above the floor do I need to install a trap and how high should the pipe exttend so that it don't back up thank you for your help

Check and see if there isn't a trap under the floor. If there is cut the cap off and use a coupling to extend the standpipe up to 36" off the floor. Good luck, Tom