If you place a weighing boat (0.229 g) on the balance, hit the tare/zero button, weigh 0.678 g of NaCl, then remove everything from the balance, then the balance will read .

The weighing balance will read  New weight = -0.229

Explanation:

From the question we are told that

    The mass of the boat is  m =  0.229 g

    The mass of the NaCl is  

Generally what the tare/zero button does is to set the reading of the weighing balance to zero , so

       when this button is pressed the balance reads zero  , now when the NaCl is placed on the balance , it reads 0.678 g , now when both the weighing boat and NaCl (i.e everything) is removed , the balance reads

    -0.229  g

The reason for this is , when the weighing boat was on the weighing balance , the tare / zero button was pressed , which made the balance to assume the weighing boat as  part of it make up hence the zero reading on the balance now when it is then removed the balance now performs a calculation like this

     New weight =  0 -  mass of the boat

=>   New weight =  0 - 0.229  

=>   New weight = -0.229  g