The cost basis of the shares would be what you paid for them. You could divide the cost basis by the number of shares acquired to come up with a cost per share, then multiply that by the number of shares you sold in 2020 as listed on your 1099-B form and that would be the cost basis of shares sold in 2020.
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Thanks. A follow up question then. I have 3 exercising events. One event qualifies those shares for QSBS. How do I enter that portion of stocks as QSBS within TurbTax? My 1099B does not identify the 3 different exercising events individually, but has the full amount listed in Box 1d.