A-Level Further Maths: C3-19 3D Matrices: Reflection in the Plane z=0

[zemba]

If the point is in (0, 1, 1) and we want to reflect it about z=0. Does it stay in (0, 1, -1) or (0, -1, -1)

[antoineblanc]

What does the matrix 1, 0, 0 0, -2, 0 0, 0, 3 represent or something similar where the values on the identity matrix are a numbers which aren’t 1