Answer:
D. 36
Step-by-step explanation:
The square of half the x-coefficient must be added. That value is (12/2)² = 36.
_____
You're trying to create a trinomial of the form ...
... (a + b)² = a² +2ab +b²
where a = x, and 2b = 12.
The number you need to add is b² = (2b/2)² = (12/2)² = 36.
