You're expected to know the basics for an AP and GP.
The term of an AP is: .
The term of a GP is: .
For the AP: .The first 3 terms of a GP are 2nd, 10th and 26th terms of an AP respectively.
Show that common ratio of GP is 2.
Since these are in GP: .
Equate  and : .
Since , we can divide by
Substitute into : .