Take an addition table from $1$ to $10$ (or any other that you like better!):

Now take a $3\times 3$ square of numbers on it, such as this red one:
Now multiply the two diagonally opposite corner numbers together:
And then find the difference between the two answers:
$5 \times 9 = 45$
$7 \times 7 = 49$
$49 - 45 = 4$
Now try the numbers in the green square:
Investigate other $3\times 3$ squares.
Now try with $2\times 2$ squares and $4\times 4$ squares.