I am wondering why this question was closed as homework.As OP said ,It was from an exam ,but I think it was not a homework type question.
The question you've linked to, when boiled down to its essential structure, basically reads
I was faced with this set-piece exercise question, and I do not know the answer. Please tell me the answer.
It is smack in the middle of the class of questions that the homework policy deems off-topic. Note that the policy holds regardless of the origin of the set-piece, so "It was from an exam" is an irrelevant aspect.
That said, I do agree that this question did contain enough of a kernel of a conceptual problem that OP could have edited the question into an on-topic form which could have been reopened $-$ but the simple fact is that OP did not do this.
Note also that there are two aspects with respect to fixing closed questions: there's a difference between who can fix such questions, and who should do so. The onus falls on the asker, and nobody else is really obligated to fix such questions. However, anybody who cares enough can fix those questions, if the OP's intent is clear enough and the edits do not alter it (with somewhat more latitude with old, abandoned questions). If you really care about this, you're perfectly welcome to edit that question into a shape that's on-topic as per the homework policy, at which time it can be reopened.