Rewrite $ 4a^{2}-49 $ as:
$$ 4a^{2}-49 = (2a)^2 - (7)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 2a $ and $ II = 7 $ , we have:
$$ 4a^{2}-49 = (2a)^2 - (7)^2 = ( 2a-7 ) ( 2a+7 ) $$