Exact area of a circle with the diameter of 16

Which of the following situations represents a linear relationship? a. theresa increases the distance she runs by 0.1 miles every week. b. the volume of a box depends on the length of box. c. the amount of money in a bank account increases by 1 percent each year. d. the population of a town doubles every 5 years.

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

Which expression is the factorization of x^2+10+21