I am 13, when i was little my hair was VERY long-almost to my knees.  Then when i was in 1st grade it got cut off to my sholders! ew! I've been growing it out for almost two years, but it's only 21 inches long! I'm taking vitamin B, trimming it every 3 months, taking biotin, brushing it with a wide-tooth comb and a boar bristle brush, and conditioning it often. what can I do to SPEED UP THE GROWTH?  ???

Unfortunately, there isn't a lot you can do to speed up growth, despite what some unethical businesses will try to tell you.  Hair grows on average a half inch per month.  Some people grow hair faster, some slower, and some have hair that grows at different rates throughout the year.  In any case, you should expect a total growth of around 6" per year, less anything you cut off during trims.  It sounds like you are right on schedule!  Keep your hair in the best condition you can and it will grow at the fastest rate at which it's capable :)