If you had the problem : x2 - 21x - 100 = 0
you would factor it this way: (x - 25)(x + 4) = 0
and the solutions are: 25, -4
Since you have: x4 - 21x2 - 100 = 0
factor it as: (x2 - 25)(x2 + 4) = 0
This can be further factored as: (x - 5)(x + 5)(x2 + 4) = 0
If you have complex numbers you can factor again: (x - 5)(x + 5)(x - 2i)(x + 2i) = 0
and the solutions are: 5, -5, 2i, -2i
If you don't have complex numbers, stop with: (x - 5)(x + 5)(x2 + 4) = 0
and the solutions are: 5, -5