Answer:
k=3
Explanation:
Given equation is

First we move -12 to the other side by adding 12 on both sides

We apply completing the square method to get square form (y-k)^2
Lets take coefficient of y and then divide it by 2
-6 divide by 2 is -3
Then we square it (-3)^2 = 9
We add 9 on both sides


Now we factor, y^2 - 6y +9 is (y-3)(y-3)= (y-3)^2

Now we compare with x^2 + (y-k)^2 = 21 and find the value of k
The value of k = 3