How can you maximize the volume of a box made from a rectangular sheet?

[Trrizzy]

Suppose someone gives you a rectangular sheet of length a and width b (so b ≤ a). You make a topless box by cutting out a square with length x out of each corner and folding up the sides. How should you cut the sheet so as to maximize the volume of your box?

 

[billy_joule]

Can you relate a, b and x to volume of the box?