1099-B Cost Basis is blank. How do I calculate it? I originally purchased 100 shares and those were converted to 2 Warrants years ago and expired in 2017.

Hi, I received a 1099-B this year.  I originally purchased 100 shares of stock in 2006 for 6.239 each, in 2012 those shares were converted to 2(two) 5 year warrants.   In 2017 those warrants expired.   How do I calculate the Cost Basis for this as the 1099-B is blank?  My account shows a loss amount for this transaction of -286.68.