[(3a-7)的2次方-(a+5)的2次方]+(4a-24)
=[（3a-7）+(a+5)]*[(3a-7)-(a+5)]+(4a-24)
=(4a-2)*(2a-12)+(4a-24)
=4(2a-1)*(a-6)+4(a-6)
=4(a-6)*(2a-1+1)
=8a(a-6)
a=4
8a(a-6)=8*4*(-2)= -64