A geometric series has first term a and common ratio r. The sum of the first two terms of the geometric series is 7.2. The sum to infinity of the series is 20. Given that r is positive, find the values of r and a. [6]

First I look at the sum to infinity and see that 20 - 20r = a

And then I put 7.2 = $\displaystyle \frac{a (1 - r^2)}{(1 - r)} $

And subbing what I worked out above, $\displaystyle \frac{20 - 20r (1 - r^2)}{(1 - r)} $

And then I end up with 7.2 = $\displaystyle \frac{20 - 20r - 20r^2 - 20r^3}{(1 - r)} $

At this point I don't know what I'm doing now. Could someone please help?