an=a1q^(n-1)
a1= 1/27
a7 = 27
(1/27)q^6 = 27
q^6 = 3^6
q = 3 or -3
an = a1q^(n-1)
= (1/27)3^(n-1) or (1/27)(-3)^(n-1)