Step 1 :
After factoring out $ -1 $ we have:
$$ -x^{2}-6x-9 = - ~ ( x^{2}+6x+9 ) $$Step 2 :
Both the first and third terms are perfect squares.
$$ x^2 = \left( \color{blue}{ x } \right)^2 ~~ \text{and} ~~ 9 = \left( \color{red}{ 3 } \right)^2 $$The middle term ( $ 6x $ ) is two times the product of the terms that are squared.
$$ 6x = 2 \cdot \color{blue}{x} \cdot \color{red}{3} $$We can conclude that the polynomial $ x^{2}+6x+9 $ is a perfect square trinomial, so we will use the formula below.
$$ A^2 + 2AB + B^2 = (A + B)^2 $$In this example we have $ \color{blue}{ A = x } $ and $ \color{red}{ B = 3 } $ so,
$$ x^{2}+6x+9 = ( \color{blue}{ x } + \color{red}{ 3 } )^2 $$