The following data are monthly existing-home sales, in millions of dwelling units, for January 2006 through June 2007. Construct a forecasting model for these data, and use it in forecasting sales for July 2007.

4.4, 4.2, 3.8, 4.1, 4.1, 4.0, 4.0, 3.9, 3.9, 3.8, 3.7, 3.7, 3.8, 3.9, 3.8, 3.7, 3.5, 3.4