Firstly find open ports of the IP (exchange ip with your machines ip address)
nmap -v -Pn ip
The relevant part of the result should look like this:
PORT STATE SERVICE 23/tcp open telnet
Use telnet to connect
Try the user name "root" with no password (hit enter). You should then be logged in.
Use the command to list all files of the directory
There should be a file called "flag.txt", which content can be displayed with
The string is the flag that needs to be submitted.