Through lounge wooden floor needs levelling

Hello I have a 3-bed semi-detached house with a through lounge. The through lounge was made, from 2 separate rooms, around 8 years ago. The issue that I have is that the wooden floor (made up of original floorboards), is at a different height slightly in the front room area to the backroom area. This is most noticeably seen in the middle of the room where the dividing wall was, and the front room floorboards are slightly higher than the backroom floorboards. I want to get this rectified and have the whole of the floor totally level so that I can put down a new carpet. The length of the room is 8 metres and the width is 3.3 metres. I am looking for some advice please to get this fixed. Thanks George.