Answer:
36
Explanation:
Find a value that is equal to the square of half of b, coefficient of x
(b/2)^2 = (12/2)^2
(6)^2
Add (6)^2 to both sides.
x^2+12x+(6)^2 =40 +(6)^2
x^2+12x+36=40+36
x^2+12x+36= 76
Factor the perfect trinomial square into (x+6)^2
(x+6)^2= 76
Thus 36 must be added to both sides to complete the square....