If f(x)=a(b)^x where f(5)=15 and f(8)=170
f(5) = 15 = a 5^x
f(8) = 170 = a8^x
divide
170 = a8^x
----------------
15 = a 5^x
34/3 = (8/5)^x
b = (8/5)
