tried solving this question but did not understand it that much

Part 1 of 2 Construct a 99.9% confidence interval for the difference in mean IQ between firstborn and secondborn sons. Let u, denote the mean IQ of the firstborn sons. Use tables to find the critical value and round the answers to at least one decimal place. A 99.9% confidence interval for the difference in mean IQ between firstborn and secondborn sons is