5    |    1    -4    -21    100    -100
     |
     |          5      5    -80     100
     |_______________________________________
          1     1     -16    20      0



2    |    1    1    -16    20
     |         2     6    -26
     ______________________________
         1     3    -13     -6


-2   |   1   1   -16   20
     |      -2     2   28
     |________________________
         1  -1    -14  48


5    |      1     1     -16    20
     |
     |            5     30
     |_________________________________
            1     6     14     nonzero


2    |    1    1    -16    20
     |         2       6   -20
     |______________________________
          1    3     -10    0

        

See my plot below, which confirms your answer.





Plot y = 


Making the plot is the most powerful and quick method to see the roots,


at least to create a hypotheses, which should be checked later by numerical calculations or/and algebraic proofs.