Step 1 :
After factoring out $ 6 $ we have:
$$ 294r^{2}-150 = 6 ( 49r^{2}-25 ) $$Step 2 :
Rewrite $ 49r^{2}-25 $ as:
$$ 49r^{2}-25 = (7r)^2 - (5)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 7r $ and $ II = 5 $ , we have:
$$ 49r^{2}-25 = (7r)^2 - (5)^2 = ( 7r-5 ) ( 7r+5 ) $$