Explanation:
that means the center of the circle is exactly in the middle between the 2 points.
and since both points have the same x value (so, the diameter here is a vertical line), it is ready to get the y value of the middle : (-7 - 0) / 2 = -3.5
so, the coordinates of the center are (3, -3.5).
and that makes the standard circle equation
(x - h)² + (y - k)² = r²
to
(x - 3)² + (y + 3.5)² = 3.5² = 12.25
as h, k are the coordinates of the center, and r is the radius (half the diameter).