You don't. Knowing two of the vector's orthogonal components doesn't
tell you what the third one is. It could be absolutely anything.
The Resultant Vector minus the other vector
If a direction is given, it is a vector. Otherwise it is a scalar.
Divide the vector by it's length (magnitude).
You cannot, unless it is a null vector. As a point.
Given a vector, speed is the magnitude of the velocity vector, |v|. Consider vector V= IVx + JVy + KVz the magnitude is |V| = ( Vx2 + Vy2 + Vz2)1/2
the magnitude and direction of the vector are given.
by method of finding resultant
If they are parallel, you can add them algebraically to get a resultant vector. Then you can resolve the resultant vector to obtain the vector components.
The zero vector is both parallel and perpendicular to any other vector. V.0 = 0 means zero vector is perpendicular to V and Vx0 = 0 means zero vector is parallel to V.
Divergence is a vector operator that measures the magnitude of a vector fields source or sink at a given point.
Velocity (vector)
resultant