hey guys, i am real stuck with this question and would like some help!

The question is:

An open top box is to be made from a square sheetof metal with a side of 12cm , by cutting a square from each corner and bending the sides up along the dashed lines.

Find the dimensions of the cut out that will result in a box of the greatest volume.

Sorry I dont have the drawing, but it is a square with 1 rectangle on each side.

Thanks