I think the trick is to add the two middle terms together. (And the 2 end terms).
8 - 12x
The variable is x. The coefficient is - 12.
8 is a constant.
The coefficient always goes with a variable.
3x^2 + 2x + 5
Nothing can be combined. The leading coefficient is 3 (which goes with the greatest power of the variable x^2 in this case) and the other coefficient is 2. 5 is a constant.