problem burning disk by disk utility

That error indicates 1 of 4 things:

1. The drive doesn't like the particular disc you inserted. Try another disc.
2. Are you using discs that are translucent? Mac OS X will often fail to burn anything to these discs, due to the way the SuperDrives are designed - they're quite picky about such things.
3. If those two fail to lead to a satisfactory answer, your optical drive might simply be dirty. This is easy to correct, even with a slot-loading drive. There are special "cleaner" discs available that you can insert into the drive, let it spin a few times, then eject it. Try again.
4. If even that doesn't work, your optical drive may be broken in such a way that it cannot burn discs. If that's the case you can buy an external burner or take your Mac in for service (cheaper if your Mac is under AppleCare extended protection still).