Calculate the area of the square (A1), and then subtract the area of the circle (A2).
Square area = side length ^2
A1 = 12^2 = 144 ft2
Area of a circle = π r^2
where:
r= radius = diameter /2 = 12/2 = 6
Replacing:
A2 = π 6^2 = 113.09 ft^2
Area of the shaded region = A1- A2 = 144-113.09 = 30.92 ft2