13 times an unknown number added to 72 equals -23 times the unknown number. what is the solution to the sentence ?

Me answer number 3plz explain how you do it

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?

What is always true about a positive integer raised to a negative odd power?