According to the information given in the exercise, the bill before the tax and tip was $13.90 and the sales tax of 6% was added to that amount.
By definition, you can write 6% as a Decimal number by dividing it by 100. Then, this is:

Let be "t" the amount (in dollars) of the sales tax.
To find the value of "t", you can set up the following equation:

Finally, evaluating, you get that this is:

Rounded to the nearest cent, this is:

The answer is: $0.83