1) 81 - (x^2 + 6x)^2 = (9 - x^2 - 6x) * (9 + x^2 + 6x) = (3 - x)(3 + x)(3 + x + 6)(3 - x - 6) = (3 - x)(3 + x)(x + 9)(-x + 3),
2) c^2 + 4c + 4 - k^2 = (c + 2)^2 - k^2 = (c + 2 - k)(c + 2 + k),
3) a^2 - b^2 - 10b - 25 = (a - b - 5)(a + b + 5).
1) 81 - (x^2 + 6x)^2 = (9 - x^2 - 6x) * (9 + x^2 + 6x) = (3 - x)(3 + x)(3 + x + 6)(3 - x - 6) = (3 - x)(3 + x)(x + 9)(-x + 3),
2) c^2 + 4c + 4 - k^2 = (c + 2)^2 - k^2 = (c + 2 - k)(c + 2 + k),
3) a^2 - b^2 - 10b - 25 = (a - b - 5)(a + b + 5).