Geometric sequence question...

Write a rule given a term and common ratio where a_4=12 and r=2.
Thanks!

a_n = a_1 r^(n-1)

a_4 = a_1(2)^3

12 = a_1(8)

12/8 = a_1

a_1 = 1.5

Now write the rule for the nth term:

a_n = 1.5(2)^(n-1)

:)