Both the first and third terms are perfect squares.
$$ 9x^2 = \left( \color{blue}{ 3x } \right)^2 ~~ \text{and} ~~ 4 = \left( \color{red}{ 2 } \right)^2 $$The middle term ( $ -12x $ ) is two times the product of the terms that are squared.
$$ -12x = - 2 \cdot \color{blue}{3x} \cdot \color{red}{2} $$We can conclude that the polynomial $ 9x^{2}-12x+4 $ 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 = 3x } $ and $ \color{red}{ B = 2 } $ so,
$$ 9x^{2}-12x+4 = ( \color{blue}{ 3x } - \color{red}{ 2 } )^2 $$