The sum of the first 3 terms of a geometric series is 171 and the common ratio is 2/3.

Yep!

$\text{well then you should know that the sum of the first three terms are}\\ a + \dfrac 2 3 a + \dfrac 4 9 a = \dfrac{19}{9}a = 171\\ a=81$

ok, thank you, Rom!

I got that answer too, I just forgot to post it...