My answer to Non-associative, non-commutative binary operation with a identity was deleted. Why/how did that happen?
Gerry, I understand your frustration and not being about post comments yourself yet on the main site. But that "answer" you gave was not an answer. Akhil Mathews (a moderator here) converted that to a comment based on that. And I just converted the edited version of that "answer" again into the comments since it is still a discussion of what the student ought to have done, and not the mathematics involved.
(I am sure you will have no problem clearing the 50 rep hurdle for comments quickly here at Math.SE.)
Also, while I understand your concern about students posting questions from homework sets on Math.SE, maybe in the future you can make some indication about the fact that you are in fact the lecturer of said course and the person who assigned the problem, and that you know the student in person? The original version of your answer comes across as somewhat demeaning without these additional information, and concerns were raised about whether it constituted "offensive content".
I suspect that if the moderators were not all familiar with your name from MathOverflow, your comment would have been summarily deleted for just being offensive and off-topic.