We can answer this question if we remember that the profit is the difference between the amount earned and the amount we spent.
Then we have the following:
1. Let p be the number of plants.
2. The amount spent per plant is $1.75.
3. The amount we will sell the plant will be $3.25 per plant.
4. Since we need to earn at least $150, we have to represent this having into account that it means that we need to earn $150 or more (>=).
Now, we can represent the situation as follows:
Since we have only one variable, p, in the inequality, we can add algebraically the coefficients as follows:
Therefore, we have to sell, at least, 100 plants to make a profit of at least $150. If we sell more than 100 plants, we will earn more than $150.