6. (-3mn^2)^3 * 2m^2 n^3 = -27m^3n^6 * 2m^2 n^3 = -54m^5 n^9
7. 24a^2 b^5 c^3 : (-0.8a^2 b^4 c) = -30bc^2
8. -0.2x^6 y^2 z - 0.5x^6 y^2 z - 0.3x^6 y^2 z = -x^6 y^2 z
9. -0.027 a^15 b^6 c^3 = (-0.3a^5 b^2 c)^3
10. 3ab^3 * (-9a^3 b) = -27 a^4 b^4
7a^4 b^4 + (-27 a^4 b^4) = -20a^4 b^4