Complicated Geometric Series Question.

The question is:

The sum of the first term and the second term of a geometric series is equal to twice the sum of the second term and third term of the series.

a] given that the common ratio is positive, find the value of the common ratio.

b] the sum to infinity of the series is 12. find, correct to two decimal places, the sum of the first eight terms of the series.

I will be able to do the part' b] OK, but the first part is troubling and I find it complicated.

I see that u1 + u2 = 2 (u2 + u3)

So [ u1 = $\displaystyle ar^{-1} $, u2 = $\displaystyle ar $] = 2 x [ u2 = $\displaystyle ar $, u3 = $\displaystyle ar^2 $ ] ...{I think}

What happens from here?

Please could someone help?