Don't just state the answer, i'd like to actually learn something. : )

The area of a circle = PI * radius^2
area = 3.14159 * 16
area = 50.265
area = PI * 16

We have to determine the area of the shaded section, which is (3 / 4) of the circle.  The unshaded section is occupied by a 90 degree angle which is (1 / 4) of a circle (All circles contain 360 degrees).
So the shaded section =
50.265 * (3 / 4) = 37.699
OR
16 * PI * (3 / 4) = 12 * PI