All you need to do is multiply 13.37 by the 6% and then Again by 7%.
It would be 13.37 • 1.06 ( The One makes an Increasing Function) Which Equals 14.17. Now Multiply the 14.17 by 1.07 and you'd get 15.16.
If you are just looking for the 6% it would equal 14.17 - 13.37 = 0.80.
That makes the Tax .80 Cents