Hi there!
In this problem we need to distribute the binomials.
(3a-5b)(a-b) A common method for distributing binomials is FOIL
3a x a = 3a^2 FOIL = First, Outer, Inner, Last
3a x (-b) = -3ab We distribute 3a into all other terms besides -5b
-5b x a = -5ab Then distribute -5b into all other terms besides 3a
-5b x -b = 5b^2 It becomes a positive since we multiply two negatives
3a^2 -3ab -5ab +5b^2 Combine like terms
3a^2 - 8ab +5b^2 Then you get a trinomial :)