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There's nothing healthy you can use to make your hair go from curly to straight unless you get a relaxer...but to get longer hair you can use a moisturizing shampoo and moisturizing conditioner...deep condition once every week....eat healthy foods...get the right amounts  of exercise...drink plenty of water.....and also try essential oils for growth as well  :)
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Section the hair into box braids and them twist them...make sure you have shea butter or a holding gel to keep the hair together so that it won't untwist :)