How do you find the opposite facing angle on a surface normal?

(This is mainly about the overwatch workshop, but if you know about normals and vectors please help!!!) What I need is to find the horizontal opposite direction from where I am looking at a surface. My friend said basically multiply the facing direction vector by negative Z component of the surface normal vector,... show more (This is mainly about the overwatch workshop, but if you know about normals and vectors please help!!!)

What I need is to find the horizontal opposite direction from where I am looking at a surface. My friend said basically multiply the facing direction vector by negative Z component of the surface normal vector, but when I run the program it seems to behave incorrectly.

I am only concerned with X and Z because I need the Y value to stay the same.
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