Investors & landlords

First, I would like to express my condolences on the passing of your mother.

Your cost basis in the stock you inherited from your Mother is the value of the stock on her date of death.  

You can find the price per share of the stock on the date of her death by going here: Historical Stock Values.  You can research what the price per share of the stock was on that day and then multiply it by the number of shares you sold.  This will be your cost basis.