At the end of each year the value of the ring will increase by 10.5%.
At the end of 1 year it will be worth 2400*(1.105).
At the end of n years it will be worth 2400*(1.105)n
You need to calculate n so that 2400*(1.105)n = 8000.
1.105n = 80/24 = 10/3.
n log 1.105 = log 10/3 which you can use a log table and/or your calculator to compute