How many numbers are there from x through 7x-8

Given f(x) = 17-x^2what is the average rate of change in f(x) over the interval [1, 5]?

Adepartment store purchases screen-printed t-shirts at a cost of $5 per shirt. they mark up the price 150% (making the selling price 250% of the store's purchase price) and put them on the sales floor. every month that a t-shirt doesn't sell, the store reduces the selling price by 25%.

What is always true when two whole numbers are subtracted?