The growth is 3 so we have to solve (1+r)^m=3 where m=number the months, r=monthly rate. 7% is assumed to be the annual rate, so the monthly rate is 7/12%=7/1200.
Taking logs of each side: mlog1.00583=log3 (any base for the logs will work) so m=log3/log(1.00583)=188.88 months approx.=15 years 9mths approx.