Step 1 :
Factor out common factor $ \color{blue}{ x } $:
$$ 49x^3-xy^2 = x ( 49x^2-y^2 ) $$Step 2 :
Rewrite $ 49x^2-y^2 $ as:
$$ \color{blue}{ 49x^2-y^2 = (7x)^2 - (y)^2 } $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 7x $ and $ II = y $ , we have:
$$ 49x^2-y^2 = (7x)^2 - (y)^2 = ( 7x-y ) ( 7x+y ) $$