My answer: (x - 7)^2 + (y - 3)^2 = (2 square root 157)^2
So a equation of a circle is:
(x-h)^2 + (y-k)^2 = r^2; (h,k) is center
(x-(7))^2 + (y-(3))^2 = r^2; place in (x,y)
((-15)-(7))^2 + ((15)-(3))^2 = r^2; subtract
(-22)^2 + (12)^2 = r^2; square
484 + 144 = r^2
628 = r^2; square root both sides
r = square root 628; simplify
r (radius) = 2 square root 157
Rewrite the equation with radius:
(x - 7)^2 + (y - 3)^2 = 2 square root 157^2;
