Explanation:
a.
the coordinates of the center of the circle are exactly in the middle between the 2 points.
since the y values are the same, this diameter is a horizontal line, and we can simply calculate the middle of the x values :
(15 - 6) / 2 = 9/2 = 4.5
and the x coordinate if the center is then 6 + 4.5 = 10.5
so, the coordinates are
(10.5, 8)
b.
the diameter is the length of the line between the 2 points
since the y values are the same, the length is simply the difference in x values : 15 - 6 = 9
c.
the radius is half of the diameter, so, 9/2 = 4.5