In order to solve this, we need to find the ratio.
We have the first term, 43, as well as 3 terms so far, with the third term being 688.
3 - 1 = 2
Set up the following formula for the ratio:
43 * ratio^2 = 688
Ratio^2 = 16
ratio = 4
43 * 4 = 172 is the second term.
Hope this helps