Nine increased by four times a number

Imagine a new pasture with grass growing on it. every day after the seeds have germinated, the number of blades of grass in the pasture triples. after 18 days, the entire pasture is completely covered in grass. how many days did it take for the pasture to be one-third covered in grass?

Can any one me with the answers to these questions asap?

Ineeeeed this is due tomorrow and i dont know the answer can you find all the exponents