【(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
=2×1／50-1
=-24/25