（3a+6）/（a²-6a+9）÷（a+2）*（9-a²）/（（a+2）（a+3））=3(a+2)/(a-3)²÷(a+2)*[-(a+3)(a-3)/(a+2)(a+3)]=3/(a-3)²*[-(a-3)/(a+2)]=-3/(a+2)(a-3)=-3/(a²-a-6)