In the problem, the mean is:

And the standard deviation:

We want to use the empirical rule to find which percentage of the data falls in the interval 96.89F - 99.61F
let's see how far from the mean are these values.
Let's add the standard deviation to the mean once:

Let's add a standard deviation again:

This is the right end of the interval. Is 2 standard deviations from the mean.
Let's do the same with the left end of the interval. If we subtract a standard deviation from the mean:

Once again:

Thus, we just saw that the interval 96.89F-99.61F is the data that is within 2 standard deviation from the mean.
The empirical rule tell us that, in a normal data set 95% of the data is within 2 standard deviations from the mean.
Thus, the percentage of adults with temperatures between 96.89F and 99.61F is 95%