S(n)=b1(q^n-1)/(q-1)
6*(3^n-1)/(3-1)=726
(6*3^n-6)/2=726
6*3^n-6=1452
6*3^n=1452+6
6*3^n=1458
3^n=1458/6
3^n=243
3^n=3^5
n=5
