Why is n=2 and not n=3 when calculating compound annual growth rate?

Thanks!

Why is n=2 and not n=3 when calculating compound annual growth rate?

Thanks!

Reading? Page No.?

Reading 33 p.173 Question 3A.

If you draw a timeline, youâ€™ll see that $9 is at time = 0 while $15.5 is at time t = 2. Question asks you to assume beginning of period.