ab＜0,则a、b异号,a＜0,所以b＞0.
因此a＜b
b-a+3＞0,a-b-9＜0.
所以|b-a+3|-|a-b-9|
=b-a+3-〔-(a-b-9)〕
=b-a+3+a-b-9
=3-9
=-6