Complete the square for x and y.
(x² +42x +21²) + (y² +38y +19²) = 47 + 21² + 19²
The equation of this circle in standard form is
(x +21)² + (y +19)² = 849
The center of the circle is at the point
(-21, -19)
Its radius is
√849
The general form of a circle centered at the origin with the same radius is
x² + y² - 849 = 0
