error when I finished lfs 7.3
 

I want to ask again.
I've finished to build lfs, and when I reboot.
in grub section, only lfs wrote, my ubuntu lost.
when I try to enter my lfs. the output like this
http://sphotos-f.ak.fbcdn.net/hphoto...09431160_n.jpg
http://sphotos-b.ak.fbcdn.net/hphoto...12146732_n.jpg
http://sphotos-c.ak.fbcdn.net/hphoto...59210094_n.jpg

and I also want to ask about the mount.
everytime i mount I always use ext4.
when I finished building lfs. and I check the partition that I use, it turns out my partition is ext3.
This is a fatal error?
do I have to start over again?

Ubuntu is still there, probably. You merely busted its boot loader by installing the LFS system's GRUB boot loader in the master boot record. It happens all the time. The book warns you about this. I personally think it's a bad idea to disturb the existing boot loader until you know that the LFS system will boot (and yours didn't). This is why the book mentions creating a boot disk for in case this happens. Another precaution against this is not to install the LFS system's GRUB boot loader in the master boot record at this time and instead add an entry for LFS in the existing boot loader. Other people probably do other things.

You probably are going to need Ubuntu back today if it was your host system. You will need your host system to fix the other things you asked about. Try downloading and creating a Super GRUB 2 Disk (easy to find with Google). Use it to boot Ubuntu (easy to learn how). Once Ubuntu is running again, repair its boot loader to get back to the situation ante-busted-boot-loader. Leave it that way. Fix the things wrong with the LFS system. For now, boot LFS with Ubuntu's boot loader or even the SG2D. Decide later what the final boot loader arrangement should be. This is my opinion anyway.

IMO, you need to recompile the kernel to enable "Maintain a devtmpfs filesystem to mount at /dev" (CONFIG_DEVTMPFS=y). This happens all the time (I did it once myself). The book has a note reminding you of this in the section on compiling the kernel. Fixing that might take care of the problems in the first and third screenshots.

For the second screenshot, IMO you merely copied the example from the book for creating /etc/sysconfig/network. The <lfs> part of that command is a placeholder for you to replace with your hostname. You didn't do that. For the same reason, you also may have botched /etc/hosts. For example, you could put localhost.localdomain as the FQDN in /etc/hosts and localhost as the HOSTNAME in /etc/sysconfig/network. See what I mean? Other people use other things for those.

Anyway, I recommend that you fix that stuff and try booting LFS again. But expect other things to be wrong. I don't mean to be so negative about this, but there is no use in pretending either. I spent days and days working through stuff like this the first time. I keep detailed notes and don't have to do this kind of stuff nowadays (that's just to throw in something positive).