(13a-8b) + (12a+9b) =

13a-8b +12a +9b =
25a +b 

(13a-8b) + (12a+9b)
= 13a.12a + 13a.9b – 8b.12a -8b.9b
=156a+117ab-96ab-72b
= 156a +21ab-72b