Old questions and answers are left on the site to provide a database of information for others. So you can't (directly) delete any of your posts, irrespecive of wheher they're questions or answers.
However if there's a special reason for deleting a particular post, such as a member who has included personal information that they'd rather wasn't permanenly online, the Editor will sometimes agree to remove a post. (Use the 'Contact Us' link, at the foot of this page).
Alternatively, if there's a good enough reason, a Moderator (such as me) might also consider removing a post.