（a²-4a+3）分之（a²-4）×（a²+3a+2）分之（a-3)-(a²-1）分之（a+3）
=(a+2)(a-2)/(a-3)(a-1) x (a-3)/(a+1)(a+2)-(a+3)/(a+1)(a-1)
=(a-2)/(a-1)(a+1)-(a+3)/(a+1)(a-1)
=(a-2-a-3)/(a²-1)
=-5/(a²-1)
=-5/(7-1)
=-5/6