First, find the mean. You'll need it to compute the variance.
[tex]\bar x=\displaystyle\frac15\sum_{i=1}^5x_i=\dfrac{21+10+6+7+11}5=11[/tex]
The variance of the sample is then computed with the formula
[tex]s^2=\displaystyle\dfrac1{5-1}\sum_{i=1}^5(\bar x-x_i)^2=\dfrac{(21-11)^2+\cdots+(11-11)^2}4=\dfrac{71}2=35.5[/tex]
The standard deviation is the square root of the variance, so
[tex]s=\sqrt{35.5}\approx5.958[/tex]
