This is a two-tailed test. An extreme event either far above the mean or far below the mean will allow us to reject the hypothesis. So we care about area over here. And over here we would put 2.5%. And over here we would have 2.5%. And we would have 95% in the middle. So we need to find this critical z-score, critical z value, and if our z value is greater than the positive version of this critical z value, then that's less probable than the odds of getting something so extreme is less than 5%.