Reflection Problem

A Given plane Ax + By + Cz = D and a point x in a room, calculate reflection of x in the plane.

Function file : function s=mirror(x, A, B, C, D)

Input : The point x and the parameters in the plane equation.

Output : The reflection s.

And so far I have wrote this:

function s = spegel(x,A,B,C,D)

normvect = [A;B;C;];

length = norm(normvect);

d = D/length;
n = normvect/length;

s = x + 2*(d - dot(x,n))*n;

end

But what value of x and parameters should I use? I am asking because the values I choose got wrong. And it should be okay to choose any value I want it? Or is the code wrong?

Regina