We can combine 19t and -6t. When we subtract these from each other, we get the following: 19t - 6t = 13t (If you don't understand this, you can see it as a story -- You have 19 apples (19a) and subtract 6 apples (6a), what's left? Answer 13t)
We can't add the seperate number (8) to this 13t, so it rests "free-standing" Hence, 8 + 13t
The variable was whether people approve or disapprove of the president. They were trying to poll just the people that live in the main part of the USA. There is slight bias that comes from excluding Alaska and Hawaii.
