6.3E1 mL or 6.3 x 10^1 mL
To write any number in sci notation. place the decimal after the first digit, then count the places from the decimal to the end of the number and write "x 10^n" or "x 10^-n" where n stands for the number of places you counted. It is +n if you moved to the right; -n if you moved to the left.
6300 is 6.3E3 (6.3 x 10^3) and
0.000634 is 6.34E-4 (6.34 x 10^-4