Answer:
200 N.
It seems very difficult but it really is not, as the wording just makes it seem difficult.
We have a force of 600 N, a force of 200N, and friction. We're leaving the direction of the forces aside. The weight of the crate is 400N, but this is totally irrelevant, as it is the crate itself, and not a force acting upon it. Now we have 600N and 200N. Taking away the 600N simply leaves us with 200N.
