Rewrite $ 9x^2-49y^2 $ as:
$$ \color{blue}{ 9x^2-49y^2 = (3x)^2 - (7y)^2 } $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 3x $ and $ II = 7y $ , we have:
$$ 9x^2-49y^2 = (3x)^2 - (7y)^2 = ( 3x-7y ) ( 3x+7y ) $$