To start, since you know the radius of a circle is half of the diameter, and the formula for the circumference of a circle is C=Dπ (d is diameter) or C=2πr, you would first find the diameter of the circle which is 8 (you find this by multiplying the radius by 2). Then, if you multiply the diameter by π or if you want to round, use 3.14. This would result in 8π or 25.12 cm as your circumference. (hope this helps XD)