4) у^3 - 18 + 6у^2 - 3у = у^3 - 3у + 6у^2 - 18 = у(у^2 - 3)+6(у^2 - 3) = (у+6)(у^2-3)
5) а^2 - ab + ac - bc = a^2 + ac - ab - bc = a(a + c) - b(a + c) = (a-b)(a+c)
6) 20а^3bc - 28ac^2 + 15a^2b^2 - 21bc = 20a^3bc + 15a^2b^2 - 28ac^2 - 21bc = 5a^2b(4ac + 3b) - 7c(4ac + 3b) = (5a^2b - 7c)(4ac + 3b)
8) 24х^2 - 44х^2у - 18х^2у^3 + 33у^4 = 24х^2 - 18х^2у^3 - 44х^2у + 33у^4 = 6х^2(4 - 3у^3) - 11у(4х^2 - 3у^3)