Whenever you can't remember Root password, you can read this tutorial and do step by step to Reset it ...
You can log in with single-user mode and create a new root password.
Reboot your computer. When GRUB is presenting the menu list, follow those instructions:
As root, changing password does not ask for your old password. Now, you can change root's password by typing:
bash# passwd root
You'll be asked to re-type the password for verification. Once you're finished, the password will be changed and you can reboot by typing shutdown -r now at the prompt; then you can log in to root as before