I hope this is the right section. If not please move.

The sum of the firsts two terms of a geometric progression is six times the magnitude of the third term:

i.e. S2 = 6*T3

This is only possible for two values of the common ratio, r = 0.5 is one value.

Calculate the other value correct to three decimal places.

Any help with this would be appreciated. If you could give a step by step explanation on how its done it would be great.