On Form 6252 Installment Sale Part 1 Gross Profit and Contract Price #8 Cost or other basis of property sold.
What information do I use to answer this question?
The last payment on this installment sale was made in 2019.
#8 needs to know how much you paid for the property when you purchased it so it can calculate the gain or loss on the sale properly.
Link to What is Form 6252 Installment Sale Income