Let the first and last term be a
As the last three terms are in AP with common difference 6, then 3rd term will be a-6 and 2nd term will be a-12.
Then the series becomes a, a-12,a-6, a
The first three term are in GP
So (a-12)^2=a(a-6)
=> a^2-24a+144= a^2-6a
=> 18a= 144
=>a = 8