Both the first and third terms are perfect squares.
$$ 64x^2 = \left( \color{blue}{ 8x } \right)^2 ~~ \text{and} ~~ 81 = \left( \color{red}{ 9 } \right)^2 $$The middle term ( $ -144x $ ) is two times the product of the terms that are squared.
$$ -144x = - 2 \cdot \color{blue}{8x} \cdot \color{red}{9} $$We can conclude that the polynomial $ 64x^{2}-144x+81 $ 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 = 8x } $ and $ \color{red}{ B = 9 } $ so,
$$ 64x^{2}-144x+81 = ( \color{blue}{ 8x } - \color{red}{ 9 } )^2 $$