first we find the mean (average)
(7 + 8 + 9 + 9 + 10 + 11)/6 = 54/6 = 9
now we subtract the mean from every data number....and square it
7 - 9 = -2...-2^2 = 4
8 - 9 = -1...-1^2 = 1
9 - 9 = 0....0^2 = 0
9 - 9 = 0...0^2 = 0
10 - 9 = 1....1^2 = 1
11 - 9 = 2....2^2 = 4
now we find the mean (average) for those numbers
(4 + 1 + 0 + 0 + 1 + 4) / 6 = 10/6 = 1.67...this is the variance
to find the standard deviation, take the sqrt of the variance
sqrt 1.67 = 1.29 <== ur answer
