where A = Final Amount = $2400
      P = Beginning Amount = $800
      R = Rate expessed as a decimal = .09
      N = Number of compoundings per year = 12
      T = Number of years = ?  (What we must find)

We substitute for all but T









 




12 years

To get the months, multiply the decimal 
part  by 12

 months

or about 12 years, 3 months.

Edwin