729x + 731y = 1508 (1)
725x + 727y = 1500 (2)
Subtract (2) from (1)
4x + 4y = 8 or x + y = 2 so y = 2 - x;
725*x + 727*(2 - x) = 1500
727*2 - 2x = 1500
2x = 727*2 - 1500
x = -23
y = 2 - (-23) = 25
x - y = -23 -25 = -48
Edited to correct silly mistake!
