According to the Empirical Rule, 68% of the data in a standard normal distribution lie within 1 std. dev. of the mean; 95% within 2 std. dev., and 99.9% within 3 std. dev. of the mean.
Here the mean is 18 and the std. dev. is 3. 68% of data lie within 18-3 and 18+3, or 15 and 21.
95% lie within two std. dev. of the mean: 18-3(2) and 18+3(2), or 12 and 24.
99% lie within three std. dev. of the mean: 18-3(3) and 18+3(3), or 9 and 27.
Summing up the areas: 95% + (half of 4%) = approximately 97%.
Approx. 97% of the data lie to the left of 24.
