8b-2a=-2(a-4b) and (a-4b)÷(-2(a-4b))=-½.
The problem reduces to -½/(a(a+b)(b+a)(b-a))=-1/(2a(a+b)²(b-a)) or 1/(2a(a+b)²(a-b)).
Or perhaps the question was: [(a-4b)/(a²+ab)][(b²-a²)/(8b-2a)]. In which case:
[(a-4b)/(a(a+b))][(b-a)(b+a)/(2(4b-a))]=[-1/a][(b-a)/2]=(a-b)/(2a).