First and foremost determine the direction in which collision is taking place. Along that direction apply the restituion concept and conservation of momentum. Remember that perpendicular to that direction there is no impulse and hence no change in velocity. In case of two spheres colliding obliquely with one of them at rest remember that the one at rest will move only along the direction of collision. I would strongly recommend against the use of any formula as there are too many subcases involved.