解(3a-2a²)-[5a-1/3(6a²-9a)-4a²]=3a-2a²-[5a-2a²+3a-4a²]=3a-2a²-(8a-6a²)=3a-2a²-8a+6a²=(6a²-2a²)+(3a-8a)=4a²-5a