Respuesta :

[tex]x^2+y^2-4x+2y=b\implies~x^2-4x+4+y^2+2y+1=b+5\implies~(x-2)^2+(y+1)^2=b+5[/tex]

This describes a circle centered at [tex](2,-1)[/tex] with radius [tex]\sqrt{b+5}[/tex].