To include a link to your original question:
Go to your original question. Right click on the web address. (This will highlight it). Select copy.
Go to your new answer. Place the cursor where you want the link to appear. Right-click and select paste.
Providing a link to the original question is the easiest way to make the reference. If you want to copy the actual text from the original question, you can highlight the relevant text by left-clicking and dragging over the text while keeping the mouse button pressed down. Then right-click and select paste. (Insert the copied text into your new answer in exactly the same way as you would a link. i.e. place the cursor at the relevant place, right-click and select paste).
Chris