SQL Query Totals Grouping Subtotals

I have a table with the following fields;

ID “primary Key”, SystemNum, PurchaseDescription, ModelNumber, Brand, Quantity, Cost

I’m trying to wright a query that returns all the fields and has a calculated field Quantity * Cost = ‘Amount’

I’ve got this far no problem it works.

SELECT [ID]
      ,[SystemNum]
      ,[PurchaseDescription]
      ,[ModelNumber]
      ,[Brand]
      ,[Quantity]
      ,[Cost]
      ,Quantity*Cost AS 'Amount'

FROM [Equipment].[dbo].[SystemsComponents]

I also need to group by SystemNum and get a total of the Amount for each group. I can’t figure out how to group it without getting an error.

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why767