The area is A = xy

We know 2x + y = 800 so y = 800 − 2x

So the area is A = x(800 − 2x) = 800x − 2x²

To maximise the area, find when dA/dx = 0

Is it a maximum?

So it is a maximum.

So the maximum area occurs when x = 200, y = 400 and that area is:

A = 200 × 400 = 80 000 m² = 8 ha