the area should be maximized. Therefore the main condition is:
Now use the length of the fence as you've done above:
Plug in this term into the equation of the area to get the characteristic function:
The graph of this function is a parabola opening down so the maximum value is at it's vertex which you can calculate by
Plug in this value into the term calculating x and you'll get x = 20.
The maximum area is 600 mē