1條F5 Math Question*

Let the first term be a, common ratio be r
The 1st term = a
The 2nd term = ar
The 3rd term = ar^2
.
.
The 6th term = ar^5

As the question mentioned...
(1) a+ar+ar^2 = 12
(2) ar^3 + ar^4 + ar^5 = -96
From (1)
a(1+r+r^2) = 12
From (2)
ar^3(1+r+r^2) = -96
(2) / (1)
ar^3(1+r+r^2) / a(1+r+r^2) = -96/12
r^3 = -8
r = -2

Put r=-2 into (1)
a(1-2+4) = 12
a(3) = 12
a=4