1099 R -filing for refund for Qualified Roth withdrawl

I purchased my first home and withdrew portions of my principle from my Roth IRA to help with the down payment.    Thus I believe this is a qualified withdrawal.  The brokerage firm withheld federal income tax and I would like to file to get a refund.  On the 1099 R form box 2b it has taxable amount not determined.  Can you walk me through what to do