Since you don't have answer choices, I'll just assume you're trying to simply 2(3g-4) - 8(g+3).
First you want to distribute the two to what is inside the parentheses and the 8 to what is inside it's parentheses:
(3g*2 - 4*2) - (8*g + 8*3)
(6g - 8) - (8g + 24)
Then, distribute the negative to the (8g+24) to get rid of the parentheses. Most people forget this part and that leads to errors!
6g - 8 - 8g - 24
Finally, simplify:
-2g - 32
The expression is equivalent to -2g-32.