We are told that the box is a cube, meaning all sides are equal.
We know that the volume is 128 cubic feet.
The volume of a cube is equal to side * side * side = side^3
So-----one side is the cube root of 128. (the answers given do not include this-----perhaps 3√128 should have been written as 3√128
√ = square root
3√ = cube root
4√ = fourth root