please help with the attached equation.

An open box is to he made from a square piece of cardboard whose sides are 13 inches long, by cutting squares of equal size from the corners and bending up the sides. Determine the size of the square that is to be cut out so that the volume may be a maximum. A square wilh a side of length |:| inches should be cut away from each comer to obtain the maximum 1volume. [Round to two decimal places as needed}