Same as yesterday:
Microsoft already answered your question: you may find an extensive tutorial dedicated to equations in this article:
- KB158918 How to Insert a Caption Next to an Equation or Object
There are several solutions posted there, for both left-, center- and right-side numbering and even a samble VBA solution. It's a little bit old, true, but basics never changed that much over the years.
You can also read through a less specific, but more up-to-date and illustrated caption tutorial. It is a general tutorial, but applies pretty well to equations - especially in pair with the KB article.