Answer:
a = 2/3
Explanation:
A cube root is the same as a 1/3 power. The square of that gives a 2/3 power.
a = 2/3
_____
Additional comment
You can see the relationship between a root index and an exponent if you consider what the root means.
Consider a cube root, for example. When you cube the root, you get the original number:
(∛x)·(∛x)·(∛x) = x
Now, let's write the root as a power of x: x^a.
(x^a)·(x^a)·(x^a) = x . . . . . where x^a = ∛x
We know this product is ...
x^(a+a+a) = x^(3a) = x^1
This tells us that ...
3a = 1 ⇒ a = 1/3
That is, ∛x = x^(1/3).
Of course an n-th root is multiplied by itself n times to get the original number, so the corresponding exponent is x^(1/n).