The equation of a circle is given as 2x^2 + 2y^2 + 4x + 12y + 6 = 0. What are the center, C, and the radius, r of the circle?
Dividing by 2 on both sides gives x^2 + y^2 + 2x + 6y + 3 = 0.
Complete the square to get (x + 1)^2 + (y + 3)^2 = 7. This is now in center-radius form. Can you proceed from here?