I am growing my hair out and it is a little longer than yours now. I got it cut to about 3 inches all over 13 months ago. Since then I take prenatal vitamins, vitamin D3 and K2 and calcium supplements and eat a well rounded diet with little processed foods. I also don't over wash my hair (every other day at most, I try to go for less) and avoid heat styling except for special occasions. I condition the bottoms/tips of my hair with extra virgin coconut oil. I watch for split ends, so that I can trim my hair at the first signs of wear, but so far I don't have any damage showing although I haven't had to trim it in over a year.
AND... of course speak with your doctor before starting any supplements. I am not a medical professional and I'm not making any recommendations for anyone else to take anything.
Avoid using heat (like curling irons, flat irons etc). These damage your hair. Also you should wash your hair frequently but not TOO much because it gets rid of natural oils that you need. You should also get healthy trims regularly. If you have a lot of split ends then your hair won't grow very much. :)