heres the eqation of a circle
(x-h) ^2 + (y - k) ^ 2 = r^2
h is the x coodinate for the radius, same for k, except its the y coordinate for the radius
if the equation is (x-5) ^ 2 + (y+1)^2 = 16
center is (5,-1) radius is 4 (really +- 4)
Try writing the equation like this:
((x-h)^2)/r^2 + ((y - k)^2)/r^2=1
This will be much easier for you when you get to conic sections, with this you can tell instantly whether it is a circle or an ellipse.
And the radius is really just 4. You can't have a negative radius.