4a^2+7ab-b^2
Step-by-step explanation:
Add the first two equations together:
3a^2-ab-2b^2
+ 2a^2+5ab-3b^2
---------------------------
5a^2+4ab-5b^2
Subtract that answer from the remaining trinomial:
5a^2+4ab-5b^2
- a^2-3ab-4b^2
--------------------------
4a^2+7ab-b^2
