【（3a-7)的平方-（a+5）的平方】/（4a-24）,
=(3a-7+a+5)(3a-7-a-5)/[4(a-6)]
=(4a-2)(2a-12)/[4(a-6)]
=4(2a-1)(a-6)/[4(a-6)]
=2a-1
=1/25-1
=-24/25