Susy the same thing happened to me in the quiz with that question, and we did it in remedials but I forgot at the quiz. What you do is the following there. There are 4 jumps between the 3rd and 7th term. So suppose the 3rd term is 8. So you write 8 x r to the power of four (becasue there are 4 jumps) = 128 (suppose 128 is the 7th term). Then divide by 8 to leave r to the power of four alone. r to the power of 4 = 128 divided by 8. You take the 4th root of 128 divided by 8, to leave r alones, the answer would be 2, so the rate is2. Then you have just to plug in the formula. So now that you know r which is 2 you can know what is t1. As t3= 8 divided by 2 is 4 which is t2 and divided by 2 is 2 whic is t1. So now just write the explicit formula for finding any term. Example: t(20) = t1 x r to the power of (n-1). t(20) = 2 x 2 to the power of 19 and you have the answer. Now with the quiz problem the difference was 1/2, you do the rest.