a+3b=0
a=-3b
原式=[(a+2b-b)/(a+2b)]÷(a+b)^2/(a+2b)(a-2b)
=(a+b)/(a+2b)×(a+2b)(a-2b)/(a+b)^2
=(a-2b)/(a+b)
=(-3b-2b)/(-3b+b)
=-5b/(-2b)
=5/2