So your teacher is right. If you do out the sum by hand it is -6 + 12 - 24 + 48 - 96 + 192 - 384 = -258. I'm not really sure what kind of formula you were using how you did it. The formula on purple math you posted is the one you want to use. A1 is -6, your r is -2 and your n is 7.
[–] hyperoperation ago
So your teacher is right. If you do out the sum by hand it is -6 + 12 - 24 + 48 - 96 + 192 - 384 = -258. I'm not really sure what kind of formula you were using how you did it. The formula on purple math you posted is the one you want to use. A1 is -6, your r is -2 and your n is 7.