A circle has equation x^2+y^2-8x+2y-1=0, the radius of this circle is 3 sqrt 2.
x^2+y^2-8x+2y-1=0
x^2-8x+16+y^2+2y+1=1+16+1
(x-4)^2+(y+1)^2=18
The center of the circle is (4, -1) and the radius is sqrt 18 = 3 sqrt 2.