Help with scale factor ?

Basically I've done some maths revision and I've stumbled across a question I can't do. Enlargement I know how to do most of them but I am baffled about how I would do it if the it said enlarge by a sf of 2 and a half. Do I times the amount I across and (Up in this case) by two then times by half or times by two and then seperately times by half and add too how much I times by two, if that makes sense lol. Anyway if you don't get it could you please explain it to me, and if you could what I would do if it was minus 2 and a half.