A unit vector is simply a vector with a magnitude of 1. To get the unit vector in the direction of the vector , all you have to do is divide the components of the vector by its magnitude. This process is called normalization.
Unit vector in the direction of
To find the vector of magnitude 3 in the opposite direction of the vector , you have to find the unit vector in the direction of , which is in the opposite direction. Since the unit vector has a magnitude of 1, and you need a vector of magnitude 3, just multiply the unit vector by 3, and you'll get the answer.