Answer:
9
Explanation:
Compare your trinomial to ...
(x +a)^2 = x^2 + 2ax + a^2
You will notice that the constant (a^2) is the square of half the x-coefficient (2a).
The x-coefficient in your trinomial is 6. The square of half that is ...
(6/2)^2 = 3^2 = 9
The value of the constant is 9 to make it a perfect square.