Arithmetic and geometric Sequence and series

Determine the numbers of terms "n" in each geometric series

a1=-3, r=3, Sn= -363

just one problem so that i can understand

Sn = {a(r^n - 1)} / (r - 1)

-363 = (-3) (3^n -1) / (3 - 1)

(-363) 2 = (-3) (3^n - 1)

242 = 3^n - 1

3^n = 243

n = 5

numbers of terms "n" in geometric series a1=-3, r=3, Sn= -363 whence
-- 363 = -- 3 [3^n -- 1] / (3 -- 1) giving 3^n = 243 = 3^5 hence n = 5

Sn= a(1-r^n)/(1-r)

-363=(-3) (1-3^n)/(1-3)
121=(1-3^n)/-2
-242=(1-3^n)
-243= -3^n
243=3^n
3^5=3^n
n=5