TIP (continued): At the Password prompt, type:
YourRootUserPassword
Not this particular word, of course, but the actual Root password for the Linux computer.
Then press the ENTER key.
Notice the prompt has changed from
[yourusername@localhost yourusername]$
to
[root@localhost yourusername]#
There's now a # at the end of the prompt. This means you are now giving commands as the Root user. You must be the Root user to start MySQL.