A lot of people are telling you to check whether your idea really is novel. You should probably do that, but even if you find out that someone else has already published it, not all is lost.
If your derivation is different (hopefully simpler) and you do additional analysis (e.g. of numerical stability), you could still get a publication out of it. (Probably not in a top journal, but better than a paper mill.)
Disclaimer: I have not published any papers myself, but I have read papers that were essentially "The method presented in X et al. is useful, but we didn't get the explanation until we rediscovered it ourselves. Here is a more understandable derivation.".
If your derivation is different (hopefully simpler) and you do additional analysis (e.g. of numerical stability), you could still get a publication out of it. (Probably not in a top journal, but better than a paper mill.)
Disclaimer: I have not published any papers myself, but I have read papers that were essentially "The method presented in X et al. is useful, but we didn't get the explanation until we rediscovered it ourselves. Here is a more understandable derivation.".