If you subtract a number from a negative number, it just becomes even more negative. If you think of it as a number line from left to right, with negative numbers on the left, and positive on the right (with zero being teh middle), then if you subtract a number from another, then you move to the left a corresponding distance on that line. So -6 would be 6 spaces left of zero. If you subtract 5 from it, you move 5 more spaces to the left, and end up at -11.