The mean of a normally distributed dataset is 12, and the standard deviation is 2.
what ? % of the data points lies between 8 and 16.

So lets do it like this:
z = (X-Mean)/SD 
z1 = (8-12)/2 = - 2 
z2 = (16-12)/2 = + 2 
According to the Empirical Rule 68-95-99.7 
Mean more or less 2SD covers 95% of the values 
So the percentage of data points falling between 8 and 16 = 95%
I hope this can help