Let $P$ be an arbitrary point having sum of the squares of the distance from the planes $x + y + z = 0 , l x - n z = 0$ and $x - 2 y + z = 0$ equal to 9 units. If the locus of the point $P$ is $x ^ { 2 } + y ^ { 2 } + z ^ { 2 } = 9$, then the value of $l - n$ is equal to