How to arrange the equation

Question

four squares of which each side is x cm are removed from a rectangular metal sheet of dimension 20cm x 15cm at its corners to form an open box.If the volume of the box is 84 x cm3 , calculate the value of x

Answer 1

Once you take out the 4 corners from the sheet of metal, you can fold the remaining sheet into a box with dimensions $x, 15-2x, 20-2x$. So the volume of the box is $x(15-2x)(20-2x) = 84x$ (given). Divide both sides by $x$ and you have a quadratic equation.

You can divide by $x$ since you're presumably not interested in a flat box with zero volume.

Answer 2

if the box made by 4 x*x sized plates the volume will be x^3 and x=4.37951.