The population variance is found by taking the mean of all squared deviations from the mean.
[ (15 − 3)2 + (2 − 3)2 + (5 − 3)2 + (-7 − 3)2 + (0 − 3)2 ] / 5 = 51.6
The population standard deviation is found by taking the square root of the population variance.
51.61/2 = 7.2
