If 6 Joules of work are done on a 6kg box that is moved 3 meters, how much force is applied to the box?
W= Fd:
D=?
F=?
W?

You are told that 6 joules of work are done so W=6 joules. It is moved 3 meters, so d = 3 meters. Now you can calculate the force, F, using W=Fd.