Question 416512
This is an optimization problem requiring derivatives.  You are on the right track.
The next step is to maximize the area
{{{A = L*W = (800-2L)*L = 800L - 2L^2}}}
Now take the derivative and set = 0
{{{dA/dL = 0 = 800 - 4L}}}
Solving for L gives L = 200 m
And W = 800 - 2L -> W = 400 m
So the max. area = A = L*W = 200*400 = 80000 m^2