Simplifying
108s6 + 42s4 + -6s2

Reorder the terms:
-6s2 + 42s4 + 108s6

Factor out the Greatest Common Factor (GCF), '6s2'.
6s2(-1 + 7s2 + 18s4)

Factor a trinomial.
6s2((-1 + -2s2)(1 + -9s2))

Factor a difference between two squares.
6s2((-1 + -2s2)((1 + 3s)(1 + -3s)))

Final result:
6s2(-1 + -2s2)(1 + 3s)(1 + -3s)