As I read it you can get to a root terminal when booting from the first slackware cd.
cd root@slackware then I try to mnt/dev/hda but it can't find it argh....!!! its on hda I know that
I think that you have either input the info from gbonhevi wrongly or your fstab is needing tweaked.
the command should be
mount -t filesystem-type /dev/hdaX /mnt
where filesystem-type is either ext2 ext3 or reiserfs depending on your chosen fstype, and X is the partition number of your / partition.
In my case this is "mount -t reiserfs /dev/hda6 /mnt"
Please note the directory /mnt must exist and you must be root.
Then as gbonvehi has said already