The center (h, k) is at
the midpoint of the diameter. The length of the radius "r" is
the distance between the center and an endpoint of the diameter.
Once you have computed the value of "r" and the location of (h, k), plug these into the circle equation, (x - h)
^{2} + (y - k)
^{2} = r
^{2}.
If you get stuck, please reply showing how far you have gotten. Thank you!