In the following expression

$\displaystyle 3\sqrt[3]x(6y-2x)= 3\sqrt[3]x(6y)-3\sqrt[3]x(2x)=

18\sqrt[3]{xy}-6x\sqrt[3]x$

I do not understand how to decide which number ends up under the radical sign.

For instance in the first term $\displaystyle 3\sqrt[3]x(6y)=

18\sqrt[3]{xy}-6x\sqrt[3]x$ why does the y get distributed under the radical?

Where as in the second term $\displaystyle 3\sqrt[3]x(2x)=6x\sqrt[3]x$ the x goes outside the radical instead of under like the above part of the expression. The author does not explain this process and its leaving me very confused.