How to Flag an Answer
A question cannot be "flagged" until it has been answered. After a question
has been answered, an option entitled "Flag" will appear in the blue tools menu (to the left of the page) under the "Question tools" section. Then, you can choose from 7 options to click to add the flag, and/or add a comment with it.
(A click on the Related link will take you to a video about "Flagging" from the Answers.com Video Help Series.)