=[(2a-b)(a-b)/(a+b)(a-b)-b(a+b)/(a+b)(a-b)]×(a+b)/(a-2b)
=(2a²-3ab+b²-ab-b²)/(a+b)(a-b)×(a+b)/(a-2b)
=2a(a-2b)/(a-b)×1/(a-2b)
=2a/(a-b)