Explanation:
x² + y² + 2x + 6y = 39
( x + 1)² - 1 + ( y + 3)² - 9 = 39
( x + 1)² + ( y + 3)² - 10 = 39
(x + 1)² + (y + 3)² = 49
radius= √49 = 7
center = ( -1 , -3)
x² + y² - 4x - 2y + 1 = 0
(x - 2)² - 4 + (y - 1)² -1 + 1 = 0
(x - 2)² + (y - 1)² = 4
radius = √4 = 2
center = (2 , 1)