My mother passed in 2017. I kept up her co-op apt maintenance since she left it to me I'm her will - I never moved into the unit and sold it in 2021. Tax implications?

In 2021, I sold my mother's NYC (Queens) co-op unit that she left to me in her will when she passed in 2017. I never moved into the place nor transferred it to my name - I was still deciding what to do in 2019 and finally put it up for sale - then COVID hit and it stayed on the market for two years.

I'm unsure what the value was in 2017 when she passed but I'm sure it was MORE than what I sold for. How do I retroactively prove cost basis?