I'll get you started:
In order for x to be a linear combination of y and u, it's second component should be zero. So x=(a,0,c) for some real numbers a and c. Now express x as a linear combination of v and w, and solve for a and c.
Let 'a' be the required vector.
you need to satisfy box[a,y,u]=0 and box[a,v,w]=0; where box[a,y,u] is the scalar triple product of a, y and u.
Since span of two given vectors is a plane, 'a' lies on the intersection of two planes hence 'a' is the vector along the line of intersection of the two planes.