Could you try these;
// stop greedy search by "?" while using .*
RewriteRule ^([a-zA-Z0-9]+)/(.*?)-(.*?)\.html /subject/content.php?ID=$2
// or more better regexp, afak \w & \d will wolk too
RewriteRule ^([\w\d-]+)/(\d+)-([\w\d-]+)\.html /subject/content.php?ID=$2
// if not set as RewriteBase /
RewriteRule ^([\w\d-]+)/(\d+)-([\w\d-]+)\.html subject/content.php?ID=$2
But first: RewriteEngine On
. :)