Question 976975
f(x) = x^3 – 21x – 20 
I always like to try x = 1  or -1 as a first step
f(-1) = -1 + 21 - 20 = 0, and
(x + 1) is a factor of f(x)
now we can apply synthetic division
-1 | 1   0  -21  -20 |
note: the 0 represents the x^2 coefficient since there is no x^2 in f(x)
|       -1    1   20 |
|    1  -1  -20    0 |
the result is
x^2 -x -20
this polynomial can be factored
(x-5)(x+4)
therefore the complete factorization is
(x+1)(x-5)(x+4) and
our zeros are -1, 5, -4