Verified answer

an = a1*q^(n-1)

6561 = 3*3^(n-1)

3^8 = 3*3^(n-1)

3^7 = 3^(n-1)

7 = n-1

n = 8 --> nº de termos

S = (an*q-a1)/q-1

S = (6561*3-3)/3-1

S = 9840 --> soma

6561=3*3^n-1

6561/3=3^n-1

2187=3^n-1

3^7=3^n-1

n-1=7

n=7+1

n=8

são 8 termos

sn=3(6561-1)/3-1

sn=3(6560)/2

sn=3*3280

sn=9840