The participants in a weight-loss class lost the following weights (in pounds) after three weeks of dieting:3,6,0,3,0,0

Assuming that these weights constitute an entire population, find the standard deviation of the population. Round your answer to at least two decimal places.

(If necessary, consult a list of formulas.)
I am coming up with the answer 21.0 but my study partner says it is still wrong can someone help me