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Hi Lydia --
Yes, as you say, it is a coin from ancient Greece with goddess Athena on the front and the owl of Athens on the back.
Find out if it is a drachm or tetradrachm using the information on this page:
https://coinquest.com/cgi-bin/cq/coins.pl?coin=10322
If the weight and diameter match those on the above page, take it to a jeweler and have it tested for silver content. If it is silver, then it is certainly genuine and the values on the above page apply. If it is not silver, it is a fake and is worth about zero.
The one in my picture is a tetradrachm. It sold for $1100 US dollars during a 2022 auction. It is in better condition than your coin. So, if your coin is genuine, it would be worth less than $1100, probably about $700.