How to export DISPLAY as a root user on RHEL7

[sam@master ~]$ xhost +
[sam@master ~]$ xauth list
[sam@master ~]$ echo $DISPLAY

[sam@master ~]$ sudo su -
[root@master ~]# xauth list
[root@master ~]# xauth list server:  MIT ..cookie.. dd2....
[root@master ~]# xauth list

[root@master ~]# firefox http://mywebsite.com