Both the first and third terms are perfect squares.
$$ x^4 = \left( \color{blue}{ x^{2} } \right)^2 ~~ \text{and} ~~ 36 = \left( \color{red}{ 6 } \right)^2 $$The middle term ( $ 12x^2 $ ) is two times the product of the terms that are squared.
$$ 12x^2 = 2 \cdot \color{blue}{x^{2}} \cdot \color{red}{6} $$We can conclude that the polynomial $ x^{4}+12x^{2}+36 $ 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 = x^{2} } $ and $ \color{red}{ B = 6 } $ so,
$$ x^{4}+12x^{2}+36 = ( \color{blue}{ x^{2} } + \color{red}{ 6 } )^2 $$