12.5663706144 or yes, 12.57
The formula to find circumference is 2(3.14)r
For this problem, first you take half of the diameter to get radius. Then, putting this into the formula, you get 2(3.14)(6) = 37.699
To find the other endpoint with a given endpoint and midpoint, you subtract the midpoint values from the endpoint, then with whatever the answer is, you subtract that from the midpoint.