Answer:
(x+3)², if looking for x, X = -3
Explanation:
X²+6x+9 is a perfect square as you immediately can see that 3+3 = 6 and 3*3 = 9.
I used the generic rectangle to solve it, 9 into top right corner and then x² onto the bottom left, multiplying eachother to get 9x² which is formed by 3x and 3x multiplied so you would have (x+3) (x+3).
I'll have a picture.