USE MY OLD IMAC AS SECONDARY DISPLAY

I have iMac 2009 (eaerly 2009) (serial# W8***OTG, model: iMac 9,1) and I want to use it as a secondary display monitor for my Macbook Air (Version 10.12.6) with MACOS Sierra). Is this possible and what cable I need to use? Please give me specific Thunderbolt cable or Mini Displayport cable. I don't see Thunderbolt and/or Mini DisplayPort on my iMac so I don't know if it's possible. thanks

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