If x + 1, x - 5, and x - 2 are in a geometric progression, then there is some constant r for which
x - 5 = r (x + 1)
==> r = (x - 5) / (x + 1)
and
x - 2 = r (x - 5)
==> r = (x - 2) / (x - 5)
Then
(x - 5) / (x + 1) = (x - 2) / (x - 5)
Solve for x :
(x - 5)² = (x - 2) (x + 1)
x ² - 10x + 25 = x ² - x - 2
-9x = -27
x = 3
It follows that the ratio between terms is
r = (3 - 5) / (3 + 1) = -2/4 = -1/2
Now, assuming x + 1 = 4 is the first term of the G.P., the n-th term a(n) is given by
a(n) = 4 (-1/2)ⁿ⁻¹
The sum of the first 12 terms - denoted here by S - is then
S = 4 (-1/2)⁰ + 4 (-1/2)¹ + 4 (-1/2)² + … + 4 (-1/2)¹¹
Solve for S :
S = 4 [(-1/2)⁰ + (-1/2)¹ + (-1/2)² + … + (-1/2)¹¹]
(-1/2) S = 4 [(-1/2)¹ + (-1/2)² + (-1/2)³ + … + (-1/2)¹²]
==> S - (-1/2) S = 4 [(-1/2)⁰ - (-1/2)¹²]
==> 3/2 S = 4 (1 - 1/4096)
==> S = 8/3 (1 - 1/4096)
==> S = 1365/512
