The total cost of the 3 shirts including shipping is $71.83. They want to know how much each shirt costs without shipping so you are going to do $71.83 subtract $7.99. This gives you $63.84 for the 3 shirts without shipping. Now you are going to take $63.84 and divide by 3. This comes out to be $21.28 For each shirt.
I was just solving that equation out for you just in case. What you need to put in the box is this:
3s + 7.99 = 71.83