Given as the side length of big square is 18 cm

Suppose the side length of each small square be a.

By cutting a square from each corner and folding up the flaps we will get a cuboidal box with

Length, L = 18 – 2a

Breadth, B = 18 – 2a and

Height, H = a

Assuming, the volume of box, V = LBH = a (18 – 2a)^{2}

Condition for the maxima and minima is

Thus, a = 3 will give maximum volume.

And the maximum volume, V = a(18 - 2a)^{2} = 432 cm^{3}