Evaluate 7 sigma notation n=1 2 (-2) ^n-1

This is a geometric of the form:

a (n) = 2 (-2) ^ (n-1)

And the sum of any geometric sum is:

s (n) = a (1-r^n) / (1-r) in this case:

s (n) = 2 (1 - (-2^n)) / (1--2)

s (n) = 2 (1 - (-2^n)) / 3 so

s (7) = 2 (1 - (-2^7)) / 3

s (7) = 2 (1--128) / 3

s (7) = 2 (129) / 3

s (7) = 86