Construct the margin of error and confidence interval

A market research firm supplies manufacturers with estimates of the retail sales of their products from samples of retail stores. Marketing managers are prone to look at the estimate and ignore sampling error. An SRS of 17 stores this year shows mean sales of 85 units of a small appliance, with a standard deviation of 7.4 units. During the same point in time last year, an SRS of 11 stores had mean sales of 93.362 units, with standard deviation 14.6 units. A decrease from 93.362 to 85 is a drop of about 10%.
Construct a 95% confidence interval estimate of the difference μ1−μ2, where μ1 is the mean of this year's sales and μ2 is the mean of last year's sales.
The margin of error is