I'm guessing you mean f(x)=15,000(9/8)^x. If this is what you mean, the population would increase by about 12,000 (12030.4870605 to be exact).
Starting equation: f(x)=15,000(9/8)^x
You can clean up the 9/8 to be 1.125
Now what you want to do is find the answer to (9/8)^5 which is 1.8020324707
Next multiply 1.8020324707 by 15,000 and you get 27030.4870605
Finally 27,030.4870605 - 15,000 gives you 12030.4870605. Which means that the population increased by about 12,000.
What is the equation???
It is linear as y increases 11 grids with every increase in x grid.
I will make a linear equation from two coordinates: (1,11),(2,22)
slope: → →
the find equation using:
y - y1 = m(x - x1) → y - 11 = 11(x-1) → y = 11x ....this is linear equation.
Therefore it's linear.
Then check if 3rd and 4th coordinate matches the equation.
y = 11x y = 11x
y = 11(3) y = 11(4)
y = 33 ...therefore matches y = 44...therefore matches
So, its a linear equation.