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 =

And subbing what I worked out above,

And then I end up with 7.2 =

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