I may have made a mistake in copying the numbers into the post.
A negative exponent tells you how many times to divide by the number, so...
(1/3) (a^3) (b^-1) correct YES
But the expression was actually \(\displaystyle \dfrac{a(2/b)}{3/a}\)
a(2/b)
3/a
a(2/b)
3a^-1 ........... turned the fraction in the denominator into a negative exponent
3a(2/b)
a^-1 ............ multied whole expression by 3 This changes the value. You can't switch a factor from denominator to numerator.
3a^2(2/b)
1 ................. divided a^-1 from denominator into a in the numerator
But I'm still left with 2/b. Not sure what negative exponent to use there. Let me know if I've made a mistake.