What is the radius of a circle whose equation is x2 + y2 + 8x – 6y + 21 = 0?

units

The equation of the circle with center (a,b) and radius r is given by the following equation:
  (the stadard form)

So we write the given equation again, in the standard equation of the circle form and then easily identify a, b, and r



collect the x'es and y's together





complete the square











so the center is (-4, 3) and radius r=2