In the lower right hand corner of the box to the persons reply you'll see 3 boxes one with a (quote) a (+) and a (lightning bolt) click on the one with the (quote symbol) and then below a box like this one will appear with the senders reply to what you said or just there answer to your question.
Then in the box just left click after the reply and add your reply and hit submit reply and you'll be good to go.
(BP)