There are more than one pieces of information required to determine the year of a boat, not just the serial number. However, given that number, it is likely that the boat is from the 2000s.
Model year 1990
what year is 7.5hp mercury comet lightening with serial # 3492076
it is a 1977 motor
6420800
1983
2003
1980
its a 1966
1978
1980
1982 Regards
1975