You can put this solution on YOUR website! THE PROBLEM MEANT:
If 6400 cubes fit in (AND completely fill) the bottom of a box,
AND the box has a square bottom,
the side of the bottom of the box is squares long,
because <--->
THE ALTERNATE INTERPRETATIONS:
IF the 6400 fit in the bottom, but you could add some more cubes there, because you still can see parts of the bottom of the box without any cubes, then the box is bigger than that, but we have no clue as to how much bigger.
IF the bottom of the box is not square, it would have two different length measurements: a length and a width.