If the first term was n=0, it would be -27+xn (with x being a number). Seeing how it goes down by 9 every time, we would have -27-9n. However, there is no option for this, so it would be wrong and we move on to the first number being x=-1. Since it goes down by 9 every time you go up, if you go down to x=0 you add 9 to go to -18. The pattern is still there, so it's -18-9n