即3a-2=b+3=0
a=2/3,b=-3
原式=b/(a-b)+b³/a(a-b)²×[-(a+b)(a-b)/b(a+b)]
=b/(a-b)-b²/a(a-b)
=b(a-b)/a(a-b)
=b/a
=-3/(2/3)
=-9/2