Why the moment arms in x-direction are considered as -x?
In your drawing, force is defined positive when upwards. By convention, pitching moment is defined positive in aircraft nose up direction. Origin of the moment is the wing leading edge, so a positive force yields a nose down moment which is negative.

Horizontal forces result in a nose up moment when above the origin, and nose down when below the origin.

That is assuming:


Positive is up, for force and position.
Positive is aft, for both force and position.
Positive is nose up, for moments.


That is the sign convention for which the equation in the book holds. The book should state the sign convention it is using, perhaps you can post that as well.