If you know the center is at (-1,3), then find r by writing out the standard formula for the equation of a circle, as follows:
(x-2)^2 + (y-5)^2 = r^2
We still have the other point to deal with: (-4,1). Subst. -4 for x in this equation, and 1 for y:
(-4-2)^2 + (1-5)^2 = r^2. Then 36 + 16 = r^2,
and 52 = r^2, so that the radius is positive sqrt(52).
You could write that as sqrt(4)sqrt(13) = 2sqrt(13). Take your pick.
