Question 1194716
What radius?  
Some part or parts of your description are missing or wrong.


The equation for the GIVEN circle, after completing-the-squares is {{{(x-3)^2+(y+2)^2=13-k}}}, so the radius for the given circle is  {{{sqrt(13-k)}}}.



If you want the a circle with center (3,7) and circumference 8pi, then
{{{(x-3)^2+(y-7)^2=r^2}}} and you would find this "r" according to 
{{{2pi*r=8pi}}}
{{{2r=8}}}
{{{r=4}}}
and the equation then is {{{(x-3)^2+(y-7)^2=4^2}}}.   This IS NOT RELEVANT to your given equation.