What he (probably) means is that the following query returns the same result as your query:

    select
        sp.Name,
        sum(s.Amount) as TotalSales
    from
        Sales s
        RIGHT join SalesPerson sp on
            sp.SalesPersonKey = s.SalesPersonKey
    group by
        sp.Name
    order by
        TotalSales desc

EDIT: Formatting.