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