Here is a second way:
the two given points are equidistant from the center (x, 0)
Use distance formulas for the two points and the center
(x-0)^2 + 4^2 = (x-1)^2 + 8^2
solve for x = 49/2 so circle center is 49/2 , 0
use one pont to solve for r in the circle formula (0,4 )
( 0 - 49/2)^2 + (4-0)^2 = r^2
r^2 = 616.25 r = sqrt (616.25)