Supposing the side parallel to the museum is L, and the other two sides that we want to fence are 2W, this would L+2W.
L+2W=400
L=400-2W
Area= LW
So,
Area=(400-2W)W
Area=400W-2W^2
Since this is a quadratic equation, the graph opens downwards because -2W^2. The maximum occurs at the vertex of the parabola.
Vertex of a paraboa is given by:
x=-2/2a
remember a quadratic equation is:
ax^2+bx+c
So
W=-400/2(-2)=100
W=100, So the longest side would be L=400-2(100)=200
Area of a rectangle is LW, so the maximum area would be
A=200*100=2000 ft^2