What is the coefficient of the variable in the expression 6-4x-8x+2

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.