(x-h)^2+(y-k)^2=r^2, where (h,k) is the center of the circle and r is the radius.
We are given two endpoints of the diameter, so the midpoint of these points is the center of the circle. (2,8) and (2, -4) so the midpoint of the circle is:
((2+2)/2, (8-4)/2)
(2,2)
The radius is just half the length of the diameter. The length of the diameter is 8--4=12 so the radius is 6 units, so our circle equation is:
