I've flagged this post with the 'other' option for flags, and described it as 'Homework question'. The post was closed as a homework question, and my flag was declined. The justification was that I did not use a standard flag, and was referred to What is flagging?. In this page it is not specified which flag I should have used, and since Homework questions must be one of the most important types of flags my question is twofold: why is there no explicit explanation in that page on what to do in 'solve my homework' type of questions? What flag should I have used?
We have a custom close reason for homework-and-exercises questions, and hence there is an option to explicitly flag posts to be closed with this reason.
Users with more than 3k reputation should do so by voting to close via the close menu. As described in the help page already linked in the question users can flag posts with custom close reasons by choosing "needs improvement" in the flagging menu, then "A community-specific reason". This menu contains all custom close reasons for a site, and you should see one corresponding to homework-and-exercises questions when you open this menu on physics.SE.
Note that flagging to close with less than 3k reputation only enqueues a question for close review, it does not cast a close vote on the question, so the choice of reason at this point isn't actually all that relevant in the first place.