I presume you left the job long time back. Got the account slips while in service, but the same are not available with you now. In that case, you may approach your ex-employer and ask for the account number. If this is not possible due to any reason, you may still submit the form for withdrawal to the ex-employer who has to fill up the missing info and send to the EPFO. If he fails to do so, you may complain to the RPFC concerned.