And that sample mean, maybe it's 15.2, could be viewed as a sample from this distribution, as a sample from the sampling distribution. So what z is, z is a random variable where you're taking n samples from this distribution up here, this population distribution, taking its mean. Then you're taking m samples from this population distribution up here, taking its mean, and then finding the difference between that mean and that mean. So it's another random variable. But what is the distribution of z? What is going to be the distribution of z? So let's draw it.