Hey all. Im new here but I thought that you might be able to help me figure out what I am doing wrong here.

Im doing Geometric Progressions for my final school year, and where on holidays (got some homework). Anyway Im stuck at the end of an excercise. Here is one of the questions.

Find the value ofnif the sum ofnterms of the series 11 + 33 + 99 + ... is equal to 108251.

Ok I get that a=11, r=3, and Sn=108251

And you put this into

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

But I just do something wrong from there.

Could someone help please ?

Thankyou very much.