Question 61071
find the center and the radius of the circle:
{{{x^2 + y^2 - 12x-16y-21 = 0}}}

Add 36 to both sides: {{{x^2-12x+36 + y^2 - 12x-16y-21 = 36}}}
Rewrite x-expression: {{{(x-6)^2 + y^2-16y-21 = 36}}}
Add 64 to both sides: {{{(x-6)^2 + y^2-16y+64 -21 = 36+64=100}}}
Rewrite y-expression: {{{(x-6)^2 + (y-8)^2 - 21 = 100}}}
Add 21 to both sides: {{{(x-6)^2 + (y-8)^2 = 121}}}

The center of the circle is (6,8).
The radius = {{{sqrt(121) = 11}}}.