There are two ways to do this.
The first is to is let the database engine calculate a new field that combines the two fields and then report on that. On the data tab, add a new field to the calc tab that adds the subtotal and tax amounts. Then on the report side, you can add a label to the header, DBText to print the combined amount for each transaction, and then a DBCalc that does a sum of the new field at the bottom.
The other option is to add a label to the header, a variable in the detail section that adds the SubTotal and the Tax amounts from the table, and then another variable in the footer that adds the values from the TotalSubtotal and the DBCalc8 (which prints the total tax) DBCalc fields.