Hello. I was reading through this pdf: https://arxiv.org/pdf/physics/0402024.pdf on relativistic momentum derivation, and I just can't seem to derive equation 3.9, which is an equation describing the relativistic velocity equation. According to the paper it is supposed to be easy, and apparently follows from equation (3.6). I tried finding the reference but I cannot find it. Could someone explain how they derive that from 3.6? Thank you.