given 3; 7; −1; 2; 0; 5; −3; 1; −1; 6; 5; −2

If you mean what I think you mean then.

 

Find the mean and the standard deviation on your regular calculator.

 

The score you want will be = mean - 2.2* SD

 

Hopefully this will be one of the scores in your list of scores.

 

 

If this causes dificulties for you, you can tell us why and maybe we can help you more.  :)