How can I execute a cmd on a startup ??
 

I am running tests on 2 Linux systems (RedHat 8.0). One system
remotely logins into another for testing. I am trying to find a way to
run

export=DISPLAY=x.y.x.y:0.0 command everytime my system reboots.

Is there a file in Linux where I can specify to run the above evertime
it reboots ??

You could put it at the end of rc.local but note that only subprocesses will have access to the variable.

If you want to make it available in all shells then put it at the end of /etc/bashrc.