[(2a-b-b)/(a-b)]÷(a-2b)/(a-b)
=2(a-b)/(a-b)·(a-b)/(a-2b)
=2(a-b)/(a-2b)
[1-1/(x+4)]÷(x²-9)/(x+4)
=(x+3)/(x+4)·(x+4)/[(x+3)(x-3)]
=1/(x-3)