The equation for a circle is x^2+y^2=r^2 so the radius is the square root of 16 which is 4. Then you take y-2 and set it equal to 0. This gives you y=2 as the y coordinate of the center. You then set x+4 equal to 0 which gives you x=-4 which is the x coordinate of the center. So the radius is 4 and the center is at (-4,2)
