ab² + (a – 1)b -1

= ab² + ba – b – 1

= (ab² + ba) – (b + 1)

= ab (b + 1) - 1(b + 1)

= (b + 1)(ab – 1)

So we get,

ab² + (a – 1)b -1 = (b + 1)(ab – 1)