Hint: Each one of the factors in the function represent one of the dimensions of the box. All of the dimensions of the box must be positive numbers (that means no zeros or negatives). On the one end of your domain interval, x has to be greater than zero. Now all you have to do is find the largest value x can be so that none of the dimensions are either zero or negative.
John
My calculator said it, I believe it, that settles it