I am 22.My vitamin b12=76.Vitamin d=10.I am taking methylcobalmin 750 mg daily. And vit d tablets too. Do I need to take vit B12 injections too?

Maybe... Wow, pretty low levels. I imagine you're taking 750 mcg daily, not mg. Increase to 1000 mcg 2x/day and chew the tablets- see http://bit.Ly/1cbhjvn some people will be able to get adequate B12 this way while others will need injections, depending on their ability to absorb b12-if levels not higher in a month get shots. For d, with levels that low try 10, 000 iu of d3/day: see http://bit.Ly/1gnxnk0.