When the box IS on the shelf 2m above the ground,
its potential energy is
(weight) x (height) = (3 N) x (2 m) = 6 joules .
THAT's the work you have to do, to lift the box up to there.
Best way that I study is first get in a comfortable room. Get the material you are suppose to be studying and a blank note book. Go through what you think you know then take what you struggle with a write it in the notebook. After writing it a few times different ways find what helps you rember it the most.
This measure of the total amount of matter an object contains is called mass. The unit used for mass is the kilogram or kg for short.
I hope this helps!