Fedora

# Operating System
uname -a
# Are you root or a named user
who am i
# What Shell you are using
echo $SHELL
# What Editor will the "crontab -e" command use
echo $EDITOR
# What Editor might the "crontab -e" command use
echo $VISUAL
# Do we have permission to use the crontab command?
# If so, is there and existing crontab for this user and what is in it?
crontab -l

1) Review your proposed crontab entry after also reading the suggested instructions in post #2.
The environment under cron is quite different from the command line.
The proposed cron line will probably be best as a named script because you will invariably have to set
some environment variables before running "mysqldump"
(not least of which would be $PATH so you can find the program).

Though stating the obvious this cron will fire at 05:11 (early hours of the morning) 7 days a week.
11 05 * * * mysqldump -u ruth -ppassword -h localhost world > /home/ruth/backup/world_try

2) Set the environment variable "EDITOR" to your normal editor and export the variable.
# If your normal editor is "vi"
EDITOR="vi"
export EDITOR

3) Copy the root crontab in case or accident
crontab -l > /tmp/root_crontab_saved

4) Edit the crontab
crontab -e

In the editing session insert your new line into the crontab.
Whether you do this by typing it or by importing your existing file is up to you.
There will then be two distinct lines.
At the end of your editing session when you save the file the crontab will become active.