okay all you have to do is complete the square of the x and y's
so divide the coefficient of x by -2, then square it all and minus the square of the coefficient of x
hence (x-3)^2 - 9
then do the same for y
(y-4)^2 - 16
so (x+3)^2 + (y+4)^2 = 36
the radius is the root of the 36 so 6, and the centre is (-3,-4) because it's whatever would make the (x+3) or (y+4) equal to 0
