An artist paints and sells square tiles. The selling price P, in dollars, of a painted tile is a linear function of the side length of the tile s, in inches, as shown in the table below.
| Side length, s (inches) | Price, P (dollars) |
| 3 | 8.00 |
| 6 | 18.00 |
| 9 | 28.00 |
Which of the following could define the relationship between s and P ?