Answer:
8
Explanation:
Remember the phrase "power to a power means to multiply the exponents"
That is, if you have a number (call it x) raised to a power (call it b), and that whole expression is raised to a power (call it c), it's the same as that number x raised to the power of the product of those two powers.

Here's an example showing to give some intuition behind this (and a way to derive the above formula if you forget it):

Or more simply,

So in this case:
(remember a number raised the to the power of 1/2 is the square root of the number; in this case, the square root of 64 is 8)