Answer
$2,000,000m^2$
Work Step by Step
As we can see from the figure, the area enclosed is $x(4000-2x)=-2x^2+4000x$.
Let's compare $f(x)=-2x^2+4000x$ to $f(x)=ax^2+bx+c$. We can see that a=-2, b=4000, c=0. $a\lt0$, hence the graph opens down, hence its vertex is a maximum. The maximum value is at $x=-\frac{b}{2a}=-\frac{4000}{2\cdot(-2)}=1000.$ Hence the maximum value is $f(1000)=-2(1000)^2+4000(1000)=2,000,000.$