To find the cost of one shirt, we can look at the cost per quantity based on the table provided.
From the table:
- For 6 shirts, the cost is $18. Therefore, the cost per shirt is \( \frac{18}{6} = 3 \) dollars.
- For 5 shirts, the cost is $15. Therefore, the cost per shirt is \( \frac{15}{5} = 3 \) dollars.
- For 4 shirts, the cost is $12. Therefore, the cost per shirt is \( \frac{12}{4} = 3 \) dollars.
- For 3 shirts, the cost is $9. Therefore, the cost per shirt is \( \frac{9}{3} = 3 \) dollars.
In every case, the cost of one shirt is consistently $3.
Thus, the cost for one shirt is $3.