Step 1 :
Factor out common factor $ \color{blue}{ x^2 } $:
$$ 25x^4-20x^3y^2+4x^2y^4 = x^2 ( 25x^2-20xy^2+4y^4 ) $$Step 2 :
Note that the polynomial $ 25x^2-20xy^2+4y^4 $ is a perfect square trinomial, so we will use the following formula.
$$ A^2 - 2AB + B^2 = (A - B)^2 $$In this example we have $ \color{blue}{ A = 5x } $ and $ \color{red}{ B = 2y^2 } $ so,
$$ 25x^2-20xy^2+4y^4 = ( \color{blue}{ 5x } - \color{red}{ 2y^2 } )^2 $$