#### Answer

$498.9~ft^2$

#### Work Step by Step

We can find the green area in two steps:
- Find area of whole rectangle (21 ft x 32 ft)
- Find area of white semi-circle (diameter 21 ft)
Subtracting the two areas gives the green area.
Area of rectangle:
We know that the area of a rectangle is given by:
$Area=length*width$
$A=lw$
So we have:
$A=21*32=672~ft^2$
Area of semi-circle:
We know that the area of a circle is given by:
$A=\pi*radius^2$
$A=\pi r^2$
For a semi circle, the area is half of a circle:
$A=\frac{1}{2}\pi r^2$
So we have ($radius=diameter/2=21/2$):
$A=\frac{1}{2}\pi (21/2)^2$
$A=173.1~ft^2$
We subtract the two areas:
$672-173.1=498.9~ft^2$