That is the adjective form of pentagon?

A pentagon is a five-sided figure, typically found in geometry. The Pentagon in Washington D.C. is five sided and named after the polygon. A pentagon is a thing, so it would be a noun.

Some examples of sentences where the Word pentagon functions as a noun are:

"A pentagon has five sides." (subject)

"That shape is a pentagon." (Predicate Nominative; a nominative is a fancy Word for noun)

Hope that helps!