X^2/3 •5x =5x^5/3

Can someone explain how the exponent is 5/3?

Written an alternate way it is x^2/3 • 5x^3/3 (because 5x is to the power of one and in order to add fractions, you have to have the same denominator). So when you multiply the like terms, you add the exponents (2/3 and 3/3).