Given side length of big square is $18 cm$

Let the side length of each small square be $a.$

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

Length, \[L\text{ }=\text{ }18\text{ }-\text{ }2a\]

Breadth, \[B\text{ }=\text{ }18\text{ }-\text{ }2a\] and

Height, \[H\text{ }=\text{ }a\]

Assuming, volume of box, \[V\text{ }=\text{ }LBH\text{ }=\text{ }a\text{ }{{\left( 18\text{ }-\text{ }2a \right)}^{2}}\]

Condition for maxima and minima is