Answer:
6d²+5cd-3dc+8
Explanation:
So by combining like terms you need to have the same letters t the same powers in order to combine them
For the first one from 6d²+5cd-3dc+8 you can combine the 5cd and -3dc because they have like terms whereas all the other ones cant combine like 7d²-3c²+6d-c there aren't aren't terms that are to the same power and letter