X^2+(y-k)^2=21 what is the value of k

Solve for k:
X^2 + (y - k)^2 = 21
Subtract X^2 from both sides:
(y - k)^2 = 21 - X^2
Take the square root of both sides:
y - k = sqrt(21 - X^2) or y - k = -sqrt(21 - X^2)
Subtract y from both sides:
-k = sqrt(21 - X^2) - y or y - k = -sqrt(21 - X^2)
Multiply both sides by -1:
k = y - sqrt(21 - X^2) or y - k = -sqrt(21 - X^2)
Subtract y from both sides:
k = y - sqrt(21 - X^2) or -k = -sqrt(21 - X^2) - y
Multiply both sides by -1:
  k = y - sqrt(21 - X^2) or k = sqrt(21 - X^2) + y

Is it b? i’m not too sure cause i haven’t referred back to economics for quite a long time now.