'Among' is the wrong word. 'Homework' is uncountable, so you can't say 'Among the homework'. And, as Neil says, you have to use the article 'the', because it's specific homework rather than homework in general. You could say 'In the new homework you sent, there's Unit 5 Past Simple, which we've already done'. or perhaps: 'The new homework you sent includes Unit 5 etc.......'

The sentence needs the article "the" before homework. I suggest you put commas after the 5, and after the word "Simple". However, be aware that the meaning would change if you put a comma after the word "there." Without the comma, "sent there" means "to that place" (туда). (Среди домашних заданий работы, которые вы послали туда, находится Unit 5....) / / / With a comma (sent, there), the sentence would mean that among the homework you sent, you will find Unit 5. (Среди домашмих заданий, которые вы послали, находится Unit 5...) In other words, the comma determines whether you are saying "there" (туда), or "there is" (есть, находиться).

Please note that in the new homework you sent unit 5 past simple had been noted as already done.