If a box is pulled straight across the floor at a constant speed and it is pulled with a horizontal force of 48 n., what is the net force (in n) on the box.

If its speed is constant and it's moving in a straight line, THAT's "constant, uniform motion".  That can only happen when there's NO net force on the object.  

So besides the 48 N forward that the puller is giving it, there must be friction with the floor, or mosquitoes all over it trying to fly backwards, or somebody crouched down behind it pulling it backwards, or some other kinds of things going on, that all add up to 48 N BACKWARDS.

Somehow, the NET force on the box has to be zero, because of the way it's moving.

c. newton's third law of motion