The longest chord on the circle is the diameter, and the center of the circle is the midpoint of the diameter.
Use the midpoint formula to find the center
the x-coordinates are the same, so you can just subtract the y-coordinates to find the radius.
10.5 - 8 = 2.5
The equation of the circle is (x - h)² + (y - k)² = r² where r is the radius and (h, k) is the center.
(x - 4)² + (y - 8)² = 6.25
