I'm assuming that you're asking about
[tex] \sqrt[5]{4x^2}\cdot\sqrt[5]{4x^2} [/tex]
If this is the case, you need to use the following rule:
[tex] \sqrt[a]{b}\cdot\sqrt[a]{c} = \sqrt[a]{bc} [/tex]
In other words, if the roots have the same order, the multiplication of the roots is the same thing as the root of the product of their content.
So, we have
[tex] \sqrt[5]{4x^2}\cdot\sqrt[5]{4x^2} = \sqrt[5]{4x^2 \cdot 4x^2} = \sqrt[5]{16x^4} [/tex]
