Since this clearly doesn't require any complex 'context', the only reason I can think of for it getting closed is that there was no effort shown. However, I believe no such rule is observed in this site since I regularly see questions with no attempt upvoted to high heaven. So, why was the question closed? I don't particularly care for it getting closed, since it served its purpose for me, and I had accepted an answer way before it got closed. I'm just trying to understand how this site works!
The banner indicating that the question was put on hold for lack of context contains a link to the following question
Among other things it contains a detailed discussion of various forms of context. The discussion contains six bullet points.
The following two seems especially relevant to me given your question.
- You can provide some motivation to your question.
Instead of just asking us to find the roots of an equation, tell us where the equation comes from.[...]
- You can tell us where the question comes from.
If your question comes from studying a textbook, let us know which book.[...]
The question as is seems a rather unmotivated. I can understand it but I do not see why one would care about it.