If O be the origin and the coordinates of
P be (1,2,-3), then find the vector equation of the plane passing through P and perpendicular to OP
So the vector OP becomes a normal to your wanted plane.
let the equation of the plane be
x + 2y - 3z = c
but the point (1,2,-3) is on that plane, so
1 + 4 + 9 = c = 14
plane equation:
x + 2y - 3z = 14