Answer:
-a^4b + b^3 - 8c^2
Step-by-step explanation:
Sum of
3a^4b-4b^3+c^2 + 6a^4b+3b^3-6c^2
= 3a^4b + 6a^4b - 4b^3 + 3b^3 + c^2 - 6c^2
= 9a^4b - b^3 - 5c^2
Sum of
6a^4b-4b^3-4c^2 + 2a^4b+4b^3+7c^
= 6a^4b + 2a^4b - 4b^3 + 4b^3 - 4c^2 + 7c^2
= 8a^4b + 3c^2
Subtract
8a^4b + 3c^2 - (9a^4b - b^3 - 5c^2)
= 8a^4b + 3c^2 - 9a^4b + b^3 + 5c^2
= 8a^4b - 9a^4b + b^3 + 3c^2 + 5c^2
= -a^4b + b^3 - 8c^2
