(3√) = cube root
2(3√)108
In order to take anything out from under the radical, we have to find a number which appears three times when we factor out 108.
108= 2 • 2 • 3 • 3 • 3
So since 3 appears three times, we can take it out, and multiply it to any numbers on the outside (in this case 2). But since there are only two 2's under the radical, those stay.
6(3√)4
This is the most simplified version, or in other words the last choice.