What is an American Eagle coin, eagle on the front , scales of justice on the back 1 troy oz 999 no date worth?

It's not technically a coin--it's commonly called a "silver round". It's value is going to be whatever the price of an ounce of silver is at the moment, which right now is about $34.12. You'd probably get a little less if you sold it, so figure $30-$32.