This is a work energy problem and I 'm not able to slove it please help? DO we have to add vector A and B to get the displacement?

Actually this is the whole correct question :

A body is displaced form A=(2m,4m,-6m) to r vector b=(6 i cap-4 j cap + 4 k cap)under a constant force F vector =(2 i cap +3 j cap - k cap)n find the work done

Recall that work = \vec F \cdot \vec r . You have vectors for both \vec F and \vec r, so all you need is to calculate the dot product of the two vectors. Do you know how to do that?