To make sure you do this correctly, first i would suggest separating the 3 and the x
3•x^3/8
For a fractional exponent, the denominator is the root you take of the number (in this case the eight root) and the numerator is the exponent of the number under the radical.
3• ^8√x^3
Sorry that's a little messy, it's the best i can do on my phone!!
The multiplication symbol ( • ) is not necessary, I just put it there to make things more clear.
Hope this helps!