I plugged in 3 for x…

It might be a stupid question, but on exercise 6 about plugging in, I plugged in 3 for x, and none of the answer choices were correct. Letter c was the closest, but still, 3 raised to 6 equals 729; minus 9 equals 720. okay, but the result for letter c, if y=9, is 72.
So, I wanted to ask you if, on this specific exercise, the only option to get the right aswer is plugging in 2 for x.
(sorry if I said any terms in the wrong way, engish is not my first language)

I plugged in 30 for the number of desks…

For the question 8 about plugging in
I plugged in 30 for the number of desks, and so e=25 and c=48. But, it didn’t match any of the answer choices. I don’t understand what went wrong. I checked it over and over again. Pleease help aaaaa