How do you tell if a equation is linear or non linear

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?

Explain how you can find the missing fraction in 3 4/5 / (blank) 2 5/7. then find the missing fraction.

Using elimination, what is -4x+2y=-12 4x+8y=-24