In one year, earnings growth of the 500 largest U.S. corporations averaged 9.2%; the standard deviation was

Question:

In one year, earnings growth of the 500 largest U.S. corporations averaged 9.2%; the standard deviation was 3.5%.
a. It can be guaranteed that 84% of these earnings growth figures will be in what interval?
b. Using the empirical rule, it can be estimated that approximately 68% of these earnings growth figures will be in what interval?