G = 2 inches
The first shape is 6 and 12. The two shapes are similar to each other, so the other shape must be 1 and 2. 6 divided by 6 is 1 so 12 divided by 6 is 2.
I think it would be $14 per book
common ratio = a4/a3 = a3/a2
8/a3 = a3/18
(a3)^2 = 8 * 18
(a3)^2 = 144
a3 = 12 or a3 = -12
common ratio = 8/12 or common ratio = 8/(-12)
common ratio = 2/3 or common ratio = -2/3
Answer: 12 or -12