Is it ethical to post an exam question and answer to the internet?
There is an alternative that avoids the question asked here entirely and would even be more valuable to your readers. 

Think about what it is about that equation (or other question) that makes it interesting. Abstract from that to a more general question that also covers the specific case. 

Then present a way of thinking about that class of problems that will lead someone to a successful solution. Explain the thinking process. Explain any connections to similar things. Etc. 

Then, you can publish that freely. 

Note that words, but not ideas, can be bound by copyright. Take the idea and expand it. Anyone is free to do that.