[（3a-7)^2 -(a+5)^2 ]/(4a-24)
=(3a-7-a-5)(3a-7+a+5)/4(a-6)
=(2a-12)(4a-2)/4(a-6)
=4(a-6)(2a-1)/4(a-6)
=2a-1
已知a=6-b,c^2=ab-9,求a^-b +a^-c 的值
题目看不懂