Is asking how to derive an expression in a paper on topic here?
If not where should I ask?
It's not generally off-topic, but that doesn't mean that every question asking for the derivation of an expression in a paper is on-topic. Also, keep in mind closure decisions are always made individually for questions by the users voting on the questions, so everything I'm going to say here is what I think would happen, not what definitely will happen.
I think it very much depends on how you ask this question and how much information besides "How do I derive this?" you give us.
If you just post the expression and ask for its derivation without any further context, I would think that the question is likely to be closed under our homework policy both for lacking effort shown and for not asking a conceptual question.
If you just post the expression and your attempt to derive it and ask us where you went wrong it's likely to be closed as a check-my-work question.
If you post an explanation with context, your efforts to derive it, and make plausible this cannot be found easily in the standard literature of the field, I think it is likely not to be closed.