Hello, ^_^Engineer_Adam^_^!
Find the area of the largest rectangle having two vertices on the xaxis
and two vertices on or above the xaxis and on the parabola: y = 9 x² Very confusing . . . then you have a diagram with numbers like "3,0000".
What does the two vertices mean in the problem? You're kidding, right? .Vertices are "corners". Code:

*
*  *
*   +   *
* :  : *
* :  :y *
:  :
*+++*
x  x
The area of the rectangle is: .A .= .2xy
. . And we know that: .y .= .9  x²
Therefore, the area function is: .A .= .2x(9  x²) .= .18x  2x³
Go for it!