Simplify

(a - b) (a² + b² + ab) -(a + b) (a² + b² - ab)

The given expression is

(a - b) (a² + b² + ab) -(a + b) (a² + b² - ab)

= (a.a² + a.b² + a.ab-b.a²-b.b²-b.ab)-(a.a² + a.b²-a.ab + b.a² + b.b²-b.ab)

= a³ + ab² + a²b-a²b-b³-ab²-a³-ab² + a²b-a²b-b³ + ab²

= -2b³

Therefore, (a - b) (a² + b² + ab) -(a + b) (a² + b² - ab) = -2b³