It seems like you want to evaluate [tex]\sqrt[3]{64}+\sqrt[3]{8}[/tex]
If so, then this is one way to do it
[tex]\sqrt[3]{64}+\sqrt[3]{8} = \sqrt[3]{4^3}+\sqrt[3]{2^3}[/tex]
[tex]\sqrt[3]{64}+\sqrt[3]{8} = 4+2[/tex]
[tex]\sqrt[3]{64}+\sqrt[3]{8} = 6[/tex]
