Here is the passage again:
Despite Cassandra's stern warning, Alex walked over to flirt with Helen.
The most beautiful—and dangerous—woman at the party.
You chose to use Punctuation Rule 9 to fix the problem:
Main clause + , + appositive.
The correction looks like this:
Despite Cassandra's stern warning, Alex walked over to flirt with
Helen, the most beautiful—and dangerous—woman
at the party.
Well done! You deserve that cyber chocolate bar!