Haw many years ago was 300BC

300 BC means 300 years before the birth of Christ.

To know how many years ago is 300 BC, you need to add 300 to the number of years that have elapsed after 1 AD (this is because there is no year zero).

Based on this:
300 BC was (2017-1) + 300 = 2016 + 300 = 2316 years ago.