The answer is one solution. Hope this helps
I like to think of the - or under as the less likely to happen like a cat beating up a bear u don’t want to put your money on the cat so everyone puts there money on the bear making the bear the over and the cat the under
(s + 3t) * (2s - 2t) = 2s^2 - 2st + 6ts - 6t^2