Botanists studying attrition among saplings in new growth areas of forests diligently counted stems in six plots in five-year-old new growth areas, obtaining the following counts of stems per acre:

9,432 11,026 10,539 8,773 9,868 10,247

Construct an 80% confidence interval for the mean number of stems per acre in all five-year-old new growth areas of forests. Assume that the number of stems per acre is normally distributed.

Iunderstand everything so far except the standard deviation calculation for when it is unknown. I got the answer of 1,787 for standard deviation but other answers to this question online say that is incorrect and I would like to understand why and how to get the proper answer.