What is the sum of the geometric sequence −4, 24, −144, if there are 8 terms?

The formular for finding the sum of the geometric is: Sn = a1[1 - r^n] /[1 - r]
Where Sn = Number of terms in the series = 8
a1 = the first term = -4
r = common ration = a2/a1 = 24 / -4 = -6.
S8 = -4[1 - [-6]^8 / [1 - (-6)]
S8 = -4 - (1 - 1679616) / 7
S8 = -4 [-1679615] /7
S8 = 6718460 /7 = 959,780.
Therefore, the sum of the first 8 terms in the series is 959,780.

This does not make sense