How to get the value n in this way?

I don't know, but using the solver in Excel, I get:
n = 2.99858689716286

I guess you have to apply the value of Q firstly. Then you could set an efficient (i noticed they all have the same degree so it wouldn't be a problem), then you modidy the degree in order to get rid of the fraction, then equal it with the value of s.
but you could also asume that there is a geomethrical progression and aplly the rules. I'd be easier and safer this way.(and you could apply the value of Q at the end of the exercise.)