First let's solve for the rate, "b", by setting up a ratio with the two points given:
16.875/7.5=(ar^3)/(ar)
2.25=r^2
r=1.5
Now we need to solve for the initial value a using either point given...
7.5=a(1.5)^1
7.5=1.5a
a=5
So now we have solved for both variables and have a complete equation:
y=5(1.5)^x
