since the rE x W and rb xTB are not perpendicular to u , so it,s not = 0 ?No, that cross product is not zero. The dot product of u with it is zero.
If you have a triple product a.(b x c) and a is parallel to either b or c then the triple product is zero. The reason is clear: b x c is perpendicular to b and c; if a is parallel to b then a is perpendicular to b x c.
how do u knw that a is parallel to b then a is perpendicular to b x c ??