Answer:
y^2+12y+36
Explanation:
We want y^2+12y+? to become a perfect square. To achieve this, take the coefficient of the y term (it is 12) and divide it by 2 (obtaining 6). Now square this 6 and add the result (36) to y^2+12y.
You'll obtain y^2+12y+36, which is a perfect square.
Strictly speaking, if you begin with y^2+12y+?,
you should write y^2+12y + 36 - 36
so as not to change the value of y^2+12y.