The most commonly used equation for circumference is C=2πr.  Where c is the circumference, π is pi (or 3.14159...), and r is the radius.

In your equation, you are given diameter.  Remember that the radius is half of the diameter.  So, in this case, the radius, r, is half of 6, or 3 meters.

Now, substitute the radius into the equation for circumference, and you get:

C= 2π(3)

= approximately 18.84 meters

What you know have found is the distance around the circle.