[（3a-7)^2-(a+5)^2]/(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
=2*3-1
=5