15) These planes will resemble the pages of a book where the
line containing the points R, S and T are members in the binding
of the book Cartesian form
Let (x1, y1, z1), (x2, y2, z2) and (x3,  y3,  z3) be the coordinates of the points R, S and T
respectively Let (x, y, z) be the coordinates of any point P on the plane with position
vector rr