1)
-4a⁸b+12a²b³c=4a²b(3b²c-a⁶)
2)
-20x⁶y-35x⁶= -5x⁶(4y-7)
3)
7xy-y+21x-3=(7xy-y)+(21x-3)=y(7x-1)+3(7x-1)=(7x-1)(y+3)
4)
12x²+6y-2x²y-y²=(12x²+6y)-(2x²y+y²)=6(2x²+y)-y(2x²+y)=(2x²+y)(6-y)
5)
16x³-xy²=x(16x²-y²)=x(4x-y)(4x+y)
6)
a³b-25ab³=ab(a²-25b²)=a(a-5b)(a+5b)