i think youre wrong na ilagay r because r is referred to as common ratio sa geometric sequence or series, because as i analyzed this is a arithmetic series dapat common difference how do i know u ask? well to find the common ratio of a geometric sequence is a2/a1 and our a1 is 13 and a2 is 11 pag plinig ko ynah value na yan magiging 0.8461, ok so anyways,
use this formula
an=a1(n-1)d
a18=13+(18-1)-2
and that is -21
