This seems pretty simple but the book gave an unexpected answer

f(x)= x^3 +2x^2 -4x -8

0= the same as above

and

and

So that: +/- 4 and -2

This gives me (-4,0), (-2,0), and (4,0)

However the book gives the answer as {-2,2} without previously giving a similar answer to similar problems. Any guidance is much appreciated. Thanks.