I just need help with 26 b. I dont understand where the s comes from and how to get it to plug into the formula s= s^2

26. Price of Unleaded Gasoline. Data collected by the Oil Price Information Service from more than 90,000 gasoline and convenience stores throughout the U.S. showed that the average price for a gallon of unleaded gasoline was $3.28 (MSN Auto website). The following data show the price per gallon ($) for a sample of 20 gasoline and convenience stores located in San Francisco. 3.59 3.59 4.79 3.56 3.55 3.71 3.65 3.60 3.75 3.56 3.57 3.59 3.55 3.99 4.15 3.66 3.63 3.73 3.61 3.57 a. Use the sample data to estimate the mean price for a gallon of unleaded gasoline in San Francisco b. Compute the sample standard deviation