Verified answer

1)

5a^3 - 80a = 5a(a^2 - 4^2) = 5a(a - 4)(a + 4)

2)

-m^2 + 6m - 9 = - (m + 3)^2

3)

2n^3 + 54x^3 = 2(n^3 + (3x)^3) = 2(n + 3x)(n^2 - 3nx + 9x^2)

4)

ac^4 - c^4 + ac^2 - c^2 = (ac^2 - c^2)(ac^2 + c^2) + (ac - c)(ac + c) = (ac - c)(ac + c)(ac^2 + c^2) + 1(ac - c)(ac + c) = (ac - c)(ac + c)(ac^2 + c^2 + 1)