Kudos to the poster on the typeset math.
With cube roots we don't have to worry that things are positive.
[tex] \sqrt[3]{16 x^4} - \sqrt[3]{128 x} = \sqrt[3]{2^4 x^4} - \sqrt[3]{2^7 x} = \sqrt[3]{2^3 x^3 2x} - \sqrt[3]{4^3 2 x} = 2x\sqrt[3]{2x} - 4\sqrt[3]{2 x}[/tex]
[tex]= (2x-4)\sqrt[3]{2 x} [/tex]
