Can someone please help me with the middle one?

Average acceleration: i have no idea how to do this.

An airplane travels at an unknown elevation before it descends 3 miles to an elevation of 15 miles. find the elevation of the plane before its descen.

Question 5 of 10 2 points if a population is a sample of the population could be a. registered voters; all citizens o o b. baptists; all religious people o c. people who purchased shoes; all customers o d. the entire school faculty; math teachers