An inbox tray has 3 walls and an open side on one of the longer sides. determine the maximum area of the tray if all three walls total to a

Let the 2 short sides be x and the long side be y

$$\\2x+y=812\\ y=812-2x\\\\ Area(A)=x*y\\ A=x(812-2x)\\ A=812x-2x^2\\\\ \frac{dA}{dx}=812-4x\\\\ \frac{d^2A}{dx^2}=-4 \quad \cap\\ $This means that any stationary point will be a maximum$\\\\ $find stat point $\frac{dA}{dx}=0\\\\ 0=812-4x\\ 4x=812\\ x=203\\ $The tray will be of maximum are if the sides are 20.3cm by 40.6cm$$ $