Can I use iMac as a second display?

I was given my friends' old 2009 iMac Core 2 Duo 2.66 24 inch desktop. Yes it is old as dirt but I only really wanna use it to connect to my Asus laptop so I have more screen space. I was told my AppleCare that this wasn't possible but see many articles online saying it is. P.S. I don't own any other Macs or Apple products besides an iPhone and know very little about their products.

The 27 inch 2009 iMac can be used as a monitor in TDM or Targeted Display Mode. Do Internet search about how to do it with a 24 inch of that year, if possible at all.
You can remove most of the insides, keep LCD panel, and buy a kit to run it as any type system video screen. Search how. Mostly just a plug in conversion.
Saw a French guy's effort to hot wire existing AC cord and PSU, lighting converter boards, the LCD panel signal input to a DVI cable on 2004 G5 iMac units. I have a few of those, might try it. Just wire cutting and soldering. The original logic board and video chip on it were not used at all.

No. Not designed for externally supplied graphics [input]