Answer: $8.49
To solve this problem, you want to find out how much 15% of 9.99 is. To do that you have to multiply 9.99 by 0.15 (which equals 1.4985) to find out how much the shirt was discounted. Then take that number and subtract 9.99 from it which gives you 8.4915 (or $8.49 rounded)
Hope this helped you!