1)bn=b1*q^n-1⇒q^n-1=1458/2=729=3^6⇒n-1=6⇒n=7
S7=b1(q^7-1)/(q-1)=2*(23787-1)/(3-1)=2*23786/2=23786
2)bn=b1*q^n-1⇒b1*3^n-1=567⇒b1=567/3^(n-1)
Sn=b1*(q^n-1)/(q-1)
b1*(3^n-1)/2=847⇒b1=1694/(3^n-1)
1701/3^n=1694/3^n -1
1701*3^n-1701=1694*3^n
1701*3^n-1694*3^n=1701
7*3^n=1701
3^n=1701/7=243=3^5
n=5
b1=567/81=7