Notice that the points at the two extremes of the diameter they give you are both located along the line y=6, and are separated by each other the distance given by: 9-(-3) = 12. So the diameter of the circle is 12 units, and then its radius is half of that: 6 units. (r = 6).
Now the center of the circle must be located also on the line y=6, and its x position must be that of the equidistant point to the two extremes. that is: x center = 9-6 = 3
Then the center of the circle is at the point (3,6) in the plane.and its radius is r=6. With this info, we can use the standard equation for a circle of radius r and center :