I have a dynacraft next mountain ridge 26" bike & it already has a front disc brake but would I be able to add a disc brake to my rear wheel which previously had a v brake on it that I ripped off?
To add a rear disc brake you must have a frame with a disc mount. You will also need a rear wheel that has a disc compatible hub. Unless you find a really good deal on something used you will spend more than the bike is worth. If you feel the rear V brakes were insufficient, your money would be better spent getting quality shoes for the rear. Some riders claim good quality V shoes work as well as low end discs.