The second and fourth terms of an exponential sequence of positive terms are 9 and 4 respectively finding the first team
ar = 9, ar^3 = 4
divide them:
r^2 = 4/9
r = ±2/3 , but the terms are to be positive, so
r = 2/3
then in ar = 9
a(2/3) = 9
a = 27/2