Answer:
You need to by 3 shirts so that you can use your coupon.
Step-by-step explanation:
Consider the provided information.
One particular brand of shirts is on sale for 20% off the original price. Each shirt originally costs $17.99
Now calculate the price of each shirts after 20% off.
If the off is 20% that means you need to pay 80% of the original cost.
Now the 80% of $17.99 is:

The cost of each shirt after sale is $14.392
Now you also have a coupon for $10 off a $40 purchase.
We wants the total should be more than $40, Multiply the cost of each shirt with 3 gives the cost greater than $40.

Now the total cost is greater than $40 and you can use a coupon for $10
Thus, you need to by 3 shirts so that you can use your coupon.