Geometric series/geometric progression

[Elec68]

I can't figure this out for the life of me:

A geometric series exists with the third term of 8 and the sixth term of 128, what is the geometric series?

 

[Hurkyl]

Have you tried anything at all? What do you know about geometric series?

 

[HallsofIvy]

In particular, do you know the formula for the n^th term of a geometric sequence? Use that formula knowing that a₃= 8 and a₆= 128 to get two equations in the two parameters you need.