can i backup before i restore

You should be able to copy iTunes purchases from your iPad to your computer's iTunes via File > Devices > Transfer Purchases - that should copy any iTunes music, films TV shows, apps etc that aren't already on your computer. If you have music from other sources (e.g. copied from CDs) on your iPad which are no longer on your computer nor elsewhere, then you will need a third-party program to copy them back from the iPad - there are some programs listed half-way down this page which should be able to copy them back : https://discussions.apple.com/docs/DOC-3991