r=2km

The circumference of a circle is equal to Pi π times the diameter d.

π⋅(diameter)

Since the diameter d is equal to 2 times the radius r, the formula for circumference using radius is 2πr.

2π⋅(radius)

Substitute the value of the radius r=2 into the formula for the area of a circle. Pi π is approximately equal to 3.14.

2⋅π⋅2

Multiply 2 by 2.

4π km

Find the Circumference circle (2km)