s3=a1(1-q^3)/(1-q)
s6=a1(1-q^6)/(1-q)
s6/s3=(1-q^6)/(1-q^3)
(63/4)/(7/4)=(1+q^3)(1-q^3)/(1-q^3)
1+q^3=9
q^3=8
q=2
s3=a1(1-q^3)/(1-q)
7/4=a1(1-2^3)/(1-2)
7/4=7a1
a1=1/4
an=a1q^(n-1)
=1/4*2^(n-1)
=2^(n-3)