Calculate the cost basis of an apartment

Sorry for my English- How do I calculate the cost basis of an apartment I own. My dad gave it to me 3 years ago after a combination deal. My dad bought the land 60 years ago.

Combination Deal meaning -a partial sale transaction, where a land owner sells a portion of the land rights to a developer. The developer builds on the land and, as part of the agreement, the land owner will own a certain percentage of the building's units.

Thank you