I think you'd find the distance from the center to that point. That's the radius.
The standard form for the equation of a circle is:
Where the point is the center of the circle.
We are given that the center is found at (4,-8) so we have our equation so far:
Now we just need to find the radius. We have been given one point on the circle, so to find the radius we simply need to find the distance from the center of the circle to that point. Recall the distance formula:
Plug your values in to find the radius, then simply enter that into the previous equation to get your final answer.