22 I think If I worked it out right
since the diameter of the base of the cylinder is 6 feet, then its radius is half that, or 3 feet.
![\bf \textit{volume of a cylinder}\\\\ V=\pi r^2 h~~ \begin{cases} r=radius\\ h=height\\[-0.5em] \hrulefill\\ r=3\\ h=9 \end{cases}\implies V=\pi (3)^2(9)\implies V=81\pi](https://tex.z-dn.net/?f=%5Cbf%20%5Ctextit%7Bvolume%20of%20a%20cylinder%7D%5C%5C%5C%5C%0AV%3D%5Cpi%20r%5E2%20h~~%0A%5Cbegin%7Bcases%7D%0Ar%3Dradius%5C%5C%0Ah%3Dheight%5C%5C%5B-0.5em%5D%0A%5Chrulefill%5C%5C%0Ar%3D3%5C%5C%0Ah%3D9%0A%5Cend%7Bcases%7D%5Cimplies%20V%3D%5Cpi%20%283%29%5E2%289%29%5Cimplies%20V%3D81%5Cpi)
Answer:
23
Step-by-step explanation:
Basically I did it backwards & opposites:
-28 divided by -7 = 4
4 + 19 = 23
I checked btw and it worked.
Answer: 28 years
Step-by-step explanation:
Given
The equation showing the value of the bag after x years is 
If the price of the bag increased by 2.5%, from the equation, we can deduce that
Initial cost of the bag is 25
Double of the initial value is 50
Insert it in the equation

It will take 28 years