Step 1 :
After factoring out $ 3 $ we have:
$$ 3x^{2}-147 = 3 ( x^{2}-49 ) $$Step 2 :
Rewrite $ x^{2}-49 $ as:
$$ x^{2}-49 = (x)^2 - (7)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = x $ and $ II = 7 $ , we have:
$$ x^{2}-49 = (x)^2 - (7)^2 = ( x-7 ) ( x+7 ) $$