已知a=84,求代数式a-4/a^2-9/1/a-3*a^2+2a-3/a^2-8a+16的值最好有过程,没有的话直接给答案

题目写得太乱,我只能按照自己理解的大概进行解答,如果理解错误,请把题目再告诉一下
(a-4)/(a^2-9)/1/[(a-3)(a^2+2a-3)]/(a^2-8a+16)
=(a-4)(a-3)(a^2+2a-3)/[(a^2-9)(a^2-8a+16)]
=(a-4)(a-3)(a+3)(a-1)/[(a-3)(a+3)(a-4)^2]
=(a-1)/(a-4)
=83/80