Rational exponents
[tex]a^\frac{m}{n}[/tex]
work like this: the numerator is the actual exponent of the base, while the denominator is the index of the root.
In other words, we have
[tex]a^\frac{m}{n}=\sqrt[n]{a^m}[/tex]
So, in you case, we have
[tex]\sqrt[5]{x^3}=x^\frac{3}{5}[/tex]
Assuming that the question contanis a typo. If you actually mean [tex]5\sqrt{x^3}[/tex],
then you can write it as [tex]5x^\frac{1}{3}[/tex]
