It should read:
By the way, if you intend on working that out, I'd strongly suggest getting those ugly factors of 7 factored out before you even begin:
, where c = (4)(49) = 196.
One more hint - when computing a derivative of a polynomial that you'll eventually want to factor, don't expand the polynomial. Keep it together.
What I'm saying here is do not try to expand into a 5 term 4th degree polynomial in x. Keep it factored.
One final hint: You can factor