a point lies on the xz plane when it's y value is zero.
the distance from a point to a plane would be the absolute value of the coordinate not on the plane.
for example:
point (1,2,3)
distance from xy-plane sqrt(x^2+y^2+z^2)= sqrt(0^2+0^2+3^2) or simply the absolute value of the z term.
distance from yz-plane sqrt(x^2+y^2+z^2)= sqrt(1^2+0^2+0^2) or simply the absolute value of the x term.
distance from xz-plane sqrt(x^2+y^2+z^2)= sqrt(0^2+2^2+0^2) or simply the absolute value of the y term.
