The circle goes thru the given points (3,-4) And (3,5). Since the x-coordinate does not change, the diameter lies right on the vertical line x=3. Since the vertical distance between these points is 5-(-4), or 9, the radius is 9/2.
The standard eq'n of a circle is (x-h)^2 + (y-k)^2 = r^2
Let's subst. what we know: we get (x-3)^2 + (y-1/2)^2 = (9/2)^2. Note that y = 1/2 is halfway between y=-4 and y=5.