the correct question is
Yuri computes the mean and standard deviation for the sample data set 12, 14, 9, and 21. he finds the mean is 14. his steps for finding the standard deviation are below. what is the first error he made in computing the standard deviation
see the picture in the attached figure N 1
we know that
The formula for Sample Standard Deviation is indicated in the attached figure N2
in this problem
n=4 and Yuri had to divide in the formula by 3
therefore
the answer is Yuri divided by n instead of n -1