1.
-2s^3+4s^2-6s
=-2(s^3-2s^2+3s)
=-2s(s^2-2s+3)
2.
-x^2+2x-1
=-(x^2-2x+1)
=-(x-1)^2
3.
4b^2-4b+1-a^2
=(4b^2-4b+1)-a^2
=(2b-1)^2-a^2
=(2b+a-1)(2b-a-1)