I started but got stuck...

A Norman Window has the shape of a rectangle surmounted by a semicircle. It has a perimeter of 40 ft.

A) find the function that models the area of the window

I ended up with this function, but I'm not sure it's right.

x²(-3╥ -4)

----------- +20x

8

B) find the dimensions, to two decimal places, that admits the greatest amount of light.