jmc

Since $18-14=4$, the common difference in the first column of squares is 4, so the number above 14 is $14-4=10$, and the number above 10 is $10-4=6$. This is also the fourth number in the row, so the common difference in the row is $(6-21)/3=-5$.

Then the seventh (and last) number in the row is $21-5\cdot 6=-9$. In the second column, the common difference is $[(-17)-(-9)]/4=-2$, so $N=-9-(-2)=\boxed{-7}$.