Summing All Observations

$\sum x = 3 + 6 + 8 + 10 + 12 + 14 + 18 + 24 + 31 = 126$

$\text{Sum} = 126$

Dividing by Total Observations

$\text{Mean} = \frac{\sum x}{n} = \frac{126}{9} = 14$

$\text{Mean} = 14$