In this question, it is probably possible to represent it as an algebraic equation, but I think it would be easier to look at it from a different perspective. Because the Goofy Gardener (weird name, by the way) found that the pigeon plant was 1 foot high and the monster fern was 7 feet high, we can represent the height with a table.
Plant | Day 1 | Day 2 | Day 3 | Day 4 | Day 5 |
Pigeon Plant | 1 | 2 | 3 | 4 | 5 |
Monster Fern | 7 | 9 | 11 | 13 | 15 |
We can stop at day 5, because on this day, the monster fern is three times as big as the pigeon plant. So, you have your answer:
The monster fern will be three times as big as the pigeon plant in 5 days.