Ahorizontal 20.-newton force is applied to a 5.0-kilogram box to push it across a rough,
horizontal floor at a constant velocity of 3.0 meters per second to the right.

calculate the weight of the box. [show all work, including the equation and substitution with units.]

Are you sure the question is asking for the weight of the box? Sounds very much like a coefficient of friction-y sort of question to me.

Nevertheless, the weight of the box (assuming we're on Earth) is equal to w=mg so either 5 x 9.81 or 5 x 10 depending on what your qualification takes g to be.

Taking it to be 9.81 I will work out the coefficient of friction as well just incase that's what the question is really asking for:

20 = u x 49.05
u=20/49.05
u=0.41 to 2sf

It may possibly be c i think |-/

Multiply the 3 dimensions, assuming it is a rectangular cuboid, to get the volume in cm³ divide mass by volume to get density in g/cm³