In Statistics, a parameter is a characteristic or measurable factor of a population. Now, for the given question, we are asked to identify the parameter being estimated. Given the context and the available values, namely the sample mean (barx) which is 57 and the standard error which is 3, we can infer the information related to the parameter being estimated.
We know that the sample mean is a good estimation of the population mean, especially if our sampling method is accurate and doesn't introduce any bias.
Therefore, in this case, if we are taking a sample and calculating its mean, this calculated sample mean is being used as an estimator for the overall mean of the population. So, the parameter being estimated here is the population mean.
By taking a sample and calculating its properties, we aim to gain some insights into the corresponding properties of the population from which the sample was drawn. This is the fundamental principle behind statistical inference.
Hence, the parameter being estimated is the population mean.