you would need to modify the total price calculation to deduct the tax amount from the sub total, and create a tax calculation to determine the the total tax amount.
You could then create a tax charge to use the tax calculation and display the tax amount as a line item in the cart.
I'm not sure of your exact tax amount, but the totalprice calculation would be:
([Price] - ([Price] * <tax percentage>)) * [Quantity]
Create another calculation called tax:
[Price] - ([Price] * <tax percentage>)