If he grows 9 plants with 3 seed packets, he grows 9 / 3 = 3 plants with 1 seed packet. Since 1 seed packet grows 3 plants, he'll need 1 more to make the plants 12.
This justifies what answer for what question? Q & A:
Carter grew 9 plants with 3 seed packets. How many more seed packets does Carter need to have a total of 12 plants in his backyard?
1