$ abc=`grep -n '^LETTERS' file.txt | awk -F: '{print $1}'`
$ echo $abc
10

To print from line 10 to the end of the file:
$ sed -n '10,$p' file.txt

How do I add this to my script because this does not work.

$ sed -n '$abc',$p' file.txt

Any help will be appreciated.

Thanks!