8% over a year is 2% (8/4) per quarter (3 months). The compound factor is 1.02, and we have to find x where 6000*1.02^x=9000, in other words when 1.02^x=9000/6000=1.5. Take logs of each side: xlog1.02=log1.5. So x=log1.5/log1.02=20.475 quarters=5.12 years. The fraction 0.12 is less than a quarter, so the interest, strictly speaking, will not be added again until the next quarter. However, 5.12 years is one of the given options.