Ce sont de longs devoirs laborieux

I’m guessing that your question isn’t about the conjugation (est being 3rd-person singular, sont being plural) but about whether the subject of the verb is ce (singular) or de longs devoirs (plural).

The answer is that in this « tour présentatif », you treat the thing you’re talking about as the subject of the verb, even though grammatically you would expect ce to take that role.

Thus, de longs devoirs is your subject and you use sont to agree with the plural.

It all depends on whether the subject is singular or plural, if you are talking about only one exercise that you find hard, you can just say:

C’est un exercice laborieux.

But if you are talking about various exercises, we would rather use:

Ce sont de longs devoirs laborieux.