Steps:-
1) Just start your system . It will show you two lines on welcome screen. Click on upper arrow key and go to the 1st line.
2) Now press e .
3) Then go down and find a line which is starting from " linux16 " .
4) Go to the end of this line and give a space and write rd.break
5) Now press Ctrl-x , Now system is rebooting in background.
6) Now it will ask for commands.
7) Write these commands in this order
mount -o remount,rw sysroot
chroot /sysroot/
passwd root
Give password of root as you want . And follow commands for reboot.
exit
exit
Now system is rebooting. When you see the welcome screen and those two lines , please go to the 1st line and press e for edit again.
8) Go down and find a line which is starting from "linux16 " .
9) Again go to the end of this line and give space and then write enforcing=0
10) Then press Ctrl-x
11) System will show you login page . Give username as root and write your password , And now you are logged In.
12) Once you are logged in ,open the terminal and put a command-
restorecon /etc/shadow
NOW ITS DONE , YOU HACKED RHEL7 !!!