Step 1 :
After factoring out $ t $ we have:
$$ t^{5}-6t^{3}+9t = t ( t^{4}-6t^{2}+9 ) $$Step 2 :
Both the first and third terms are perfect squares.
$$ x^4 = \left( \color{blue}{ t^{2} } \right)^2 ~~ \text{and} ~~ 9 = \left( \color{red}{ 3 } \right)^2 $$The middle term ( $ -6x^2 $ ) is two times the product of the terms that are squared.
$$ -6x^2 = - 2 \cdot \color{blue}{t^{2}} \cdot \color{red}{3} $$We can conclude that the polynomial $ t^{4}-6t^{2}+9 $ 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 = t^{2} } $ and $ \color{red}{ B = 3 } $ so,
$$ t^{4}-6t^{2}+9 = ( \color{blue}{ t^{2} } - \color{red}{ 3 } )^2 $$