greatest common divisor of two polynomials f,g ? Is it unique ?

a)

Define the greatest common divisor of two polynomials $\displaystyle f,g.$ Is it unique ?

b)

Find the greatest common divisor of the polynomials f :$\displaystyle x^4 + x - 2 $and $\displaystyle g = x^2 - 1$

c)

The polynomial$\displaystyle f = x^4 - x^3 -7x^2 + x + 6 $has four real zeros, given that 1 and 3 are zeros, find the other two ?

This is a past paper without solutions, it would be great if you could help me with these to revise for my exam, thanks alot !