+230 The sequence of integers in the row of squares and in each of the two columns of squares form three distinct arithmetic sequences. What is the value of $N$?
+1223 The first vertical column can be filled from the bottom: 18, 14, 10, 6.
In the horizontal row, we have 21, ___, ___, 6. The common difference of this is 5, so we can fill that in: 21, 16, 11, 6, 1, -4, -9.
Finally, we have the vertical column to the right. From the bottom, the numbers are -17, __, __, __, -9, N. The common difference is 2, so N = -7
