(a-3b)(a2+3ab+9b2)-4a2(2a-5b)

Simplify the expression:

1. (a - 3b)(a^2 + 3ab + 9b^2) = a^3 - 27b^3 (difference of cubes).

2. -4a^2(2a - 5b) = -(8a^3 - 20a^2b) = -8a^3 + 20a^2b.

Add them: (a^3 - 27b^3) + (-8a^3 + 20a^2b) = -7a^3 + 20a^2b - 27b^3.

Final result: -7a^3 + 20a^2b - 27b^3.