Both the first and third terms are perfect squares.
$$ 9x^2 = \left( \color{blue}{ 3u } \right)^2 ~~ \text{and} ~~ 64 = \left( \color{red}{ 8 } \right)^2 $$The middle term ( $ -48x $ ) is two times the product of the terms that are squared.
$$ -48x = - 2 \cdot \color{blue}{3u} \cdot \color{red}{8} $$We can conclude that the polynomial $ 9u^{2}-48u+64 $ 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 = 3u } $ and $ \color{red}{ B = 8 } $ so,
$$ 9u^{2}-48u+64 = ( \color{blue}{ 3u } - \color{red}{ 8 } )^2 $$