SQL Query Totals Grouping Subtotals

I've been trying to use Sum() with GROUP BY but I get errors regarding the other fields that I'm not summing or grouping.

SELECT [ID]
      ,[SystemNum]
      ,[PurchaseDescription]
      ,[ModelNumber]
      ,[Brand]
          ,[Quantity]
      ,[Cost]
          ,Quantity*Cost AS 'Amount'
          ,SUM(Quantity*Cost) AS 'Total'
  FROM [Equipment].[dbo].[SystemsComponents]
  GROUP BY 
      [SystemNum]

Msg 8120, Level 16, State 1, Line 2

Column 'Equipment.dbo.SystemsComponents.ID' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

---

why767