Note that the polynomial $ 16x^2-24xy+9y^2 $ is a perfect square trinomial, so we will use the following formula.
$$ A^2 - 2AB + B^2 = (A - B)^2 $$In this example we have $ \color{blue}{ A = 4x } $ and $ \color{red}{ B = 3y } $ so,
$$ 16x^2-24xy+9y^2 = ( \color{blue}{ 4x } - \color{red}{ 3y } )^2 $$