Well, all you would have to do is divide 21 into 8, (21 divided by 8), and the remainder will become your new numerator, and the divisor, (8), will be your denominator, and the whole answer will be your whole number, so your mixed number will be 2 5/8, because 21 divided by 8 = 2, 2 times 8 = 16, 21 - 16 = 5, 2 is your whole number, 5 is your numerator, and 8 is your divisor. =)
I hope I helped! =D