need help with child theme functions.php file

I need some help setting up child functions.php file.

I created a custom functions.php file for my child them with this code inside:

<?php
add_filter( ‘private_title_format’, ‘yourprefix_private_title_format’ );
add_filter( ‘protected_title_format’, ‘yourprefix_private_title_format’ );

function yourprefix_private_title_format( $format ) {
return ‘%s’;
}
?>

when I upload this file to m child theme dir, I get this error:

Fatal error: Cannot redeclare yourprefix_private_title_format() (previously declared in /usr/local/pem/vhosts/112577/webspace/siteapps/25678/htdocs/wp-content/themes/thefarm/functions.php:8) in /usr/local/pem/vhosts/112577/webspace/siteapps/25678/htdocs/wp-content/themes/twentyeleven/functions.php on line 620

what am I doing wrong?
thanks in advance