If you check the dictionary, it gives "equity" as a noun and not an adjective. That said, I don't think most people would object to its use as an adjective in the phrase "equity investments."
I remember a footnote in a materials science textbook I had in college. It pointed out that Oxford had a school not for "Materials Science" (as other colleges did), but "The Science of Materials." Because Oxford is famous for its dictionary and they insisted that "materials" is not an adjective
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