This is my first group by and I'm trying to do the following:

1. Group by Branch, Account, and Reference Number
2. Add the grouped items together, then sum up the groups column TaxAmount and Amount as the total column.

Here is what I have but it's not compiling:

```
SELECT        SUM(SUM(Amount) + SUM(TaxAmount)) AS Total
FROM            Truck.PpsTransaction
GROUP BY ReferenceNumber, Branch, AccountNumber

```

What am I doing wrong?