Let N be an integer that is divisible by 7

Let "xy" be a number .
Eg, for 203,  x=20, y=3
    for 1984, x=198 y=4

If this number is divisible by seven, then:

10x + y = N

=> 7x + 3x + y = N

=> 3x + y = N

=> 9x + 3y = N

So

10x + y = N
9x + 3y = N

=> x - 2y = N