Both the first and third terms are perfect squares.
$$ 25x^2 = \left( \color{blue}{ 5x } \right)^2 ~~ \text{and} ~~ 16 = \left( \color{red}{ 4 } \right)^2 $$The middle term ( $ 40x $ ) is two times the product of the terms that are squared.
$$ 40x = 2 \cdot \color{blue}{5x} \cdot \color{red}{4} $$We can conclude that the polynomial $ 25x^{2}+40x+16 $ 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 = 5x } $ and $ \color{red}{ B = 4 } $ so,
$$ 25x^{2}+40x+16 = ( \color{blue}{ 5x } + \color{red}{ 4 } )^2 $$