Suppose that f(t)=Qoa^1=Qo(1+r)^t with f(4)=72.2 and f(10)=176.7. Find what a is and what r is.
a^1 = a
If you meant
f(t) = Qoa^t = Qo(1+r)^t
then clearly a = 1+r
Qo(1+r)^4 = 72.2
Qo(a+r)^10 = 176.7
Now divide and you have
(1+r)^6 = 2.447
Now finding r is easy.
After that, you can find Qo.