How to paint this surface please

[Longhotsummers]

The wall was previously wallpapered and the builder has had to plaster part of it so there is smooth plaster alongside wallpaper that has a rough surface now. I just want to paint it, nothing fancy but think the two different surfaces will look odd. I’ve attached a photo to illustrate.
Can anyone advise what to do please?

Strip the wallpaper off and then either put lining paper on the whole wall or just paint straight onto the plaster.

Thanks, but the plaster goes over the top of the wallpaper to the side of what’s showing so if I strip it, there’ll be a dip in the level between it and the plaster. Not sure what to do!

I'd either remove the paper and use a filler and sand it down or I'd fill the rough surface and sand it down. I use Toupret - it mixes well and it's easy to sand down to a smooth finish.
www.screwfix.com/p/toupret-interior-filler-2kg/390PP?tc=WA3&gad_source=1&gad_campaignid=19823510542&gbraid=0AAAAAD8IdPzbXv3EMvbe4qxA2uwdm4EiH&gclid=Cj0KCQjw2tHABhCiARIsANZzDWr3V4IKMS-kdRUOSAdT-PUZKh6Mc7iR4Uy2eOatmxWN9LLYPlwG3FUaAl7wEALw_wcB&gclsrc=aw.ds

You might get away with lining papering the whole wall and then painting it but I think you will see the plaster join throught it.

Do you mean the whole wall was plastered over paper, or just that the edge overlaps?

If the latter, then do what @Flightfromhell says. You may need a couple of passes of filler and some rubbing down, but it will blend.

If the former, your plasterer needs to be shot.