Then from this equation from because we have been u, so we have del u by del x plus del v by del y plus del w by del z equal to 0 gives us. It gives us del u by del x that will give you a and del v by del y that will give you d and w is 0. So, a plus d 0 is implies a is equal to minus d, and this is the criteria for the velocity vector given by this. To ensure that there is a fluid motion is possible, so a has to be minus d in this case for a possible fluid motion.