The 3 is part of the radical. It is asking what number is being multiplied by itself three times to get 64.
For example:
[tex] \sqrt[3]{27} [/tex]
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
