The mean is $19000 and the standard deviation is $2000 use the 68-95-99.7 rule to find the percentage of buyers who paid less than $13000

The threshold of interest ($13,000) is $6,000 or 3 standard deviations below the mean ($19,000). The rule you are asked to use tells you that 99.7% of the distribution lies within 3 standard deviations of the mean. This tells you that 0.3% lies outside. Since the distribution is symmetrical, half those values are 3 standard deviations above the mean and the half of interest are 3 standard deviations below the mean.

The percentage of buyers who paid less than $13,000 is about 0.3%/2 = 0.15%, according to the rule you are to use.