20ft 6in
The NBA introduced the three point line in the 1979-80 season at a length of 23'9" from the top of the key to a length of 22 feet in the corners. In the 1994-95 season, the NBA changed things so the...
36 feet
The NBA three point line is 23'9"; there were however a few seasons when they shortened the line to 22' in the mid 90's.
According to the NBA website, the 3-point line was adopted for the 1979-1980 season.