The points (0,4) and (1,8) lie on a circle whose center is on the x-axis. What is the radius of the circle?
... Sorry if I misunderstood, but the question looks pretty impossible to solve to me.
The line between the two points will be a chord of the circle.
So, find the equation of the line that passes through its midpoint and at right angles to it.
It will pass through the centre of the circle so find out where it crosses the x-axis etc..