Help with math word problem? 10pts

An open box is to be made out of a 12-inch by 18-inch piece of cardboard by cutting out squares of equal size from the four corners and bending up the sides. Find the dimensions of the resulting box that has the largest volume.

Going out to 13 decimal places, the answer would be:
Width x Length x Height = Area
7.2917999920579" x 13.2917999920579" x 2.3541000039711" = 228.1620727177220"