Number square? There is a number rectangle - whose answer I recollect is -5
Add up the digits on the last row (I forget what they are, so 12 for example becomes 1+2= 3) and you see a set of subtractions. Row1-Row2 = Row3
The diagonal rows of numbers are similar, although I forget exactly what they are.
Add up all the digits, eg 12 space 34 is 1+2+3+4=10 and it comes to the row below
So answer is I recollect 17 although I don't have the puzzle to hand.