We need to take account of the sample size to make an adjustment for the population standard deviation, so s, the sample deviation=2.1/√12=0.6062, where 12 is the sample size.
The difference between the sample mean and the population mean is 17.3-16.1=1.2 inches below the population mean. This is 1.2/0.6062=1.9795 standard deviations below the mean and is the Z value of -1.9795. This corresponds to a probability of 0.02388 in the normal distribution.
Therefore P(X<16.1)=0.0239 approx (2.39%).