What is the sum of the first ten terms of the sequence -82,-76,-70,-64?

To find the sum of a geometri sequence from first term to nth term you do
Sn=(n(a1+an))/2

a1=-82
n=10

hmm
common difference is 6 since adding 6 each time
first term is -82
10th term is -82+6(10-1)=-28

sum of 10 terms is

Sn=(10(-82-28))/2
Sn=(10(-110))/2
Sn=(-1100)/2
Sn=-550

answer is -550