answer question

Click on the Reply button at the side of the Message.

If the message is on a blue background, then do what Owdboggy says - it's a Private message to you alone.

If the message is on a green background, then it's a comment on one of your photos and to reply to those, you click on the link bottom right of the message where it says 'on photo such and such' which takes you to the photo and the member's comment - then you can do a response. Does this make sense?