It is 256.
You simply rewrite the given number this way,
![\sqrt[3]{ 64^4}](https://img.qammunity.org/2019/formulas/mathematics/middle-school/ug1k4rmp7jww4gqncme1pyj2enwbc7cy9d.png)
This means it is 64*64*64*64, then take the cubic root of the answer.
All can be done simply by a calculator.
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Hope this helps.