We want to get a 95% confidence interval based on our sample. We want to create an interval around this where we are confident that there's a 95% chance that this true mean, the true mean of the differences lies within that interval. And to do that, let's just think of it the other way. How can we create a 95% interval around this, around the mean, where we are 95% sure, or construct an interval around this, where we're 95% sure that any sample from this distribution, and this is one of those samples, that there's a 95% chance that we will select from this region right over here. So we care about a 95% region right over here. So how many standard deviations do we have to go in each direction? And to do that, we just have to look at a z-table.