I am not a college student so I am sorry or you could ask your friends.
The GCF is what they all have in common.
2x is the only thing they ALL share.
Follow these steps:
-8.4b - 4.2 = 12.6
Add 4.2 on both sides:
-8.4b = 16.8
Divide by -8.4 on both sides to give you the final answer of:
b = -2
So you're answer is indeed correct! Hope this helps and that the steps are clear enough!