Example$-$MAD with Historical Data

The table below provides actual sales for years $1$ through $7.$ The firm uses a three$-$year moving average to make forecasts.

$\backslash$table$[[$Year$,$Sales,Forecast$],[1,100,],[2,200,],[3,300,],[4,400,200],[5,100,300],[6,900,267],[7,800,467],[8,,]]$

a$.$ What is the forecast for year $8?$