Gulnara1979 x2+y2−6x−8y=−21+0
x2+y2−6x−8y=−21
(x−3)2−9
(x−3)2−9+y2−8y=−21
(x−3)2+y2−8y=−21+9
(y−4)2−16
(x−3)2+(y−4)2−16=−21+9
(x−3)2(y−4)2=−21+9+16
(x−3)2+(y−4)2=−12+16
(x−3)2+(y−4)2=4