MacBook Air confused

Hi,

I replaced my Macbook Air after my son spilled Orange juice on it. I was able to have the entire setup moved to my new Mac.
I had been using an external monitor with the defunct Mac, then stopped doing it for a year or so. Today, I plug the new Mac to the 21 inch monitor and...

when I go into parameters>display, my MacBook is recognized as an external screen and vice versa. That means I cannot adjust the resolution properly on the external screen.

Any idea how to correct that?

Thanks

via ehMac.ca http://ift.tt/2dNnyH5