Rule of 72 is useful if you are really bad at maths and want to know roughly in how many years your investment will double when you know the interest rate.
No of years = 72 / Rate of interest
So if the rate of interest is 9 % then 72 / 9 = 8 years in which the investment will be doubled.
or
Rate of interest = 72 / No of years
If you want to invest for 12 years and you want your investment to be double then what should be the rate of interest i.e. 72 / 12 = 6%.