I have normal vector to the surface. And I want to receive vector perpendicular to this normal vector.

This perpendicular vector has to be normalized and lie towards x and z direction as normal one.

http://oi61.tinypic.com/4lsfb8.jpg

Any ideas for 3 equations which will serve to find the solution? I have only one as on the picture, so I need to have 2 more ones.