Math ! pleaaassee evaluate the summation of 2 times negative 2 to the n minus 1 power, from n equals 1 to 7. −256 −84 84 86

The formula for the sum of a geometric sequence with first term "a", common ratio "r" and number of terms "n" is
  S = a·(r^n -1)/(r -1)
You have a=2, r=-2, n=7, so the sum is
  S = 2·((-2)^7 -1)/(-2 -1)
  = 2(-128-1)/(-3)
  = 2(43) = 86