Data:
Mean: $1500
Standard deviation: $300
Sample: 80.7
Empirical rule define the following intervals:
68.27% of the data are in: [μ - s, μ + s]
95.45% of the data are in: [µ – 2s, µ + 2s]
99.73% of the data are in: [µ – 3s, µ + 3s]
Being μ the mean and s the standard deviation:
You need to find the number of farms between $900 and $2100 per acre:
µ=1500
s=300
68.27% of the data will be in the interval:

95.45% of the data will be in the interval: [900,2100]

As the 100% of the sample is 80.7, the 95.45% is: 77.02 farms

Then, the number of farms whose land and building values per acre are between $900 and $2100 is 77.02.