Start from the a^2 term.
Cube rooting it will give you a^(2/3).
When raising the power of a^(2/3) by 6, your original index (2/3) is multiplied by 6.
2/3 x 6 = 4
As such, you now have a^4 in the denominator (bottom of the fraction).
Now, 1/(a^4) will be equal to a^-4, which is your answer.