
All posible zeros for the first exercise are: -1, + 1, -2, +2, -3, +3, -4, +4, -6, -6, -12, +12, - 5/6, +6/5, -6/5.
Synthetic division is useful to find the possible roots to find the exact roots in an quick way.
3.- Yes, it's possible. Zeros are when a polynomial crosses the x-axis. Some polynomials do not cross this axis so there will not be roots.
4.-

It isn't possible to factor this polynomial.