ohhhhh...but we dnt knw if it passes through j or not so maybe it has a j value 
consider the whole set up on the

consider an arbitrary vector and the unit vector

We assume both vectors are parallel.
Lets assume the arbitrary vector is

+

Gradient of

= Gradient of the other vector

Thus if an arbitrary
vector and

are parallel, the co-effecient of j for the arbitrary vector has to equal 0.