$($b$)$ Develop a three$-$week moving average for this time series. Compute MSE and a forecast for week $7.$ Do not round intermediate calculations. If required, round your answers to two decimal places.

$\backslash$table$[[$Week$,$Value,Forecast,$],[1,18,,],[2,15,,],[3,15,,],[4,13,,8],[5,17,,8],[6,14,,x$