hey, just need a hint if you don't mind mates!

The first 3 terms of a geometric progression are also the 1st, 9th , 11th terms of an arithmetic progression.

Given that the terms of the geometric progression are all different, find the common ratio, r.

I've started with: a + ar + ar^{2}= a + (a+8r) + (a+10r)

ar + ar^{2}- 18r= 2a

i also thought about doing a system of equations but don't really know where i'm going really

Thanks for your help