This can be done in two ways. The first is to simply place Year and Week Number on the columns shelf with sum of your measure on the rows shelf. This will cause the line to break. Another option is to create a calculated field similar to the following:
datename('week', [Order Date]) + " / " + datename('year', [Order Date])
However, this will cause the date to be a string. Right click on it and select Sort by: [Date Field]
Hope this helps!