（a+2)/(a-3)-（a-3）/(a+2)除以a^2-6a+9/a^2-4
=(a+2)/(a-3)-（a-3）/(a+2)除以(a-3)^2/(a+2)(a-2)
=(a+2)/(a-3)-(a-2)/(a-3)
=4/(a-3)