Step 1 :
Factor out common factor $ \color{blue}{ 2 } $:
$$ 50x^2-2y^2 = 2 ( 25x^2-y^2 ) $$Step 2 :
Rewrite $ 25x^2-y^2 $ as:
$$ \color{blue}{ 25x^2-y^2 = (5x)^2 - (y)^2 } $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 5x $ and $ II = y $ , we have:
$$ 25x^2-y^2 = (5x)^2 - (y)^2 = ( 5x-y ) ( 5x+y ) $$