Neither is very tough, even though the second may look more challenging at first. Try drawing the first circle, with its center at the origin and passing through (0,6). The distance from the radius to that point is 6, so you have a radius. Now you simply plug it into the circumference formula:
C = 2Πr
= 2Π(6)
= 12Π or 37.68
If you draw the center of the second circle as well as that point on it, you'll see that it has a distance of 4, and thus a radius of 4. Use the same formula and get your answer.
