Full disclosure: I did vote to close that post as being "not focused" for asking many questions, even though the main closure reason indicated is the homework / check my work reason.
should i answer a question with multiple parts...
I would say no. Questions that do not follow the site policy should not be answered. Supplying an answer indicates that you think the question is a good question and deserves an answer. It gives future posts that have similar issues a post to point at and say, "but this question like mine got some nice answers. Why can't I get similar answers on my question?"
...if i only know some parts of it?
I would say this right here is one of the reasons why posts that ask multiple questions are not encouraged. IMO If you have only answered parts of the question then you have not fully answered the question (although there isn't a rule against this). If multiple users were to answer like this, then it makes it hard for other users coming to learn from the question to find the answer, as it will be scattered across many different answers.