Respuesta :

[tex](x-h)^2+(y-k)^2=r^2\\ (h,k)-\text{ center}\\\\ x^2+y^2-6x+8y+21=0\\ x^2-6x+9+y^2+8y+16-4=0\\ (x-3)^2+(y+4)^2=4\\ \text{center}=(3,-4)\\ r=\sqrt4=2[/tex]