Найдите номер члена прогреcсии вn=972 ,если в1=4 q=3

Bn = b1 *q^(n-1)
bn/b1 = q^(n-1)
log(q)bn/b1 = log(q)q^(n-1)
log(q)bn/b1 = (n-1)
n = log(q)bn/b1 +1
n = log(3) 972/4 +1 = 6
n =6 
проверка
bn = b1 *q^(n-1) = 4 *3^(6-1) = 972