12) $x^2-kx+(k-1)$ factors easily and you can read the roots right off. Use the quadratic formula if you can't figure out the factors.

b) is trivial once you figure out (a)

13) from the quadratic formula what must be true to ensure real roots? What does that translate to here? Is it always true?

14) just solve the inequality for k. Remember the problem says "for all x"