you could evaluate it for n=4....
As romsek said, "thus" is the wrong word. It implies that the fact that 109 is not divisible by 5 follows from what went before. In fact, "what went before" shows that 109 is of the from "n^3- n^2+ n+ 4" for n= 5. The fact that it is not divisible by 5 follows from the fact that 109= 5(21)+ 4.
Is that the right way of present my answer or there is any specific notation to do this?