The 3 is part of the radical. It is asking what number is being multiplied by itself three times to get 64.
For example:
![\sqrt[3]{27}](https://img.qammunity.org/2018/formulas/mathematics/middle-school/rny4bktuf83t5qlefel4ilmrxz6iqknx.png)
What number is being multiplied by itself three times to get 27 (?)(?)(?)
(2)(2)(2)=8
(3)(3)(3)=27
The answer will be 27 for the example
For your question, 4 is being multiplied (4)(4)(4)=64