The equation of any circle is
(x - h)2 + (y - k)2 = r2
(h, k) = centerpoint of the circle
r = radius
They give you the endpoints of the diameter. Use the distance formula with the given endpoints of the diameter to find the length of the diameter.
To find the radius, divide the length of the diameter by 2.
To find the centerpoint of the circle, use the midpoint formula with the given endpoints of the diameter.
Once you have all of that, plug it into the equation of the circle.