6 to 10 10-6 = 4 that is the range
20. $158, 400 - multiply 40 by 22 to get the total square footage(880), then multiply 880 by 15 to get they total for the month, which is 13,200. lastly, you take 13,200 multiplied by 12(for each month in a year), and you get $157,400.
Critical points is where the derivative (slope) is zero or does not exist. So to do this we have to find the derivative of our function:

So we apply chain rule:
=

Set our first derivative to zero and solve for x:
3(x^2 - 1) * 2x = 0
So we can see that (by plugging in) 0, -1 and 1 makes our solution true
So our critical value is x = 0, x = -1, x = 1
Divide 71 and 5 to get a decimal since 71 does not end with 5 or 0. Do it to get 14 1/5 or 14.2