What is the case and syntax of trhe word feet in the sentence "The center of a tennis net is three feet long."

In this sentence, "feet" is an adjective modifying the noun "long."

feet is part of a dimensional adjective "three feet long". Feet modifies three, which together modifies long. The usage is as a predicate adjective.

Three feet long is the center.
The three feet long center is there.
Is the three feet long center there?

feet is part of a dimensional adjective "three feet long". Feet modifies three, which together modifies long. The usage is as a predicate adjective.

Three feet long is the center.
The three feet long center is there.
Is the three feet long center there?

In terms of case, "feet" is in the plural form in this sentence. The singular form of "feet" is "foot." The syntax of "feet" in this sentence is that it functions as an adjective modifying the noun "long." Specifically, "feet" is part of the dimensional adjective "three feet long." In this construction, "feet" modifies "three," which together modifies "long." The usage is as a predicate adjective in this sentence.