What is the radius of a circle whose equation is x squared plus y squared plus 8 x minus 6 y plus 21 is equal to 0 ?

Complete the square for each variable: x² + 8x = (x + 4)² − 16, and y² − 6y = (y − 3)² − 9. The equation becomes (x + 4)² + (y − 3)² = 4. In the standard circle equation, the right side is the radius squared, so the radius is √4 = 2 units; the correct choice is 2 units.

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