33. How many terms of the GP series will make the sum 2186?

Here in the given series we get,

First number a = 2,  Common ratio r = 3
S_n = a(rⁿ-1)/(r-1)
2186 = 2(3ⁿ -1)/(3–1)
2186 = 3ⁿ - 1, or
3ⁿ = 2187, or
therefore n = 7.
Hence, there would be 7 terms