3a+b/a^2b-ab^2+b-a/ab:a^2-b^2/3a-b

= (3a+b) / (ab (a-b)) - (a-b) / ab * (3a-b) / ((a-b) (a+b)) = (3a+b) / (ab (a-b)) - (3a-b) / (ab (a+b)) = (3a²+3ab+ab+b²-3a²+ab+3ab-b²) / (ab (a-b) (a+b)) = 8ab / (ab (a-b) (a+b)) = 8 / (a²-b²)