Please help in Proving of volume.

An open water tank with a square base x m, and a height y m has a total surface area of 5m^2.

Show that V = 1/4 ( 5x - x^3)

V = 1/4 ( 5x - x^3)
Surface Area = x² + 4xy = 5
=> 4xy = 5 - x²
=> y = ( 5 - x² ) / 4x ( this is Height.)
Now V = area of the base x height = ( x² ) height
=> V = ( x² ) [( 5 - x² ) / 4x ]
=> V = (5x² - x^4) / 4x
=> V = (5x²/4x) - (x^4/4x)
=> V = (5x/4) - (x^3/4)
=> V = 1/4( 5x - x^3)
Answer