Forces:
Box--> W(weight) and T(tension)
Rope-->T1(reaction of T) and T2(because of the helicopter)
So first i calculated Weight:
W=mg=400*10=4000N
In order to find the acceleration i should use Newton's 2nd law so:
(Box) : T - W = ma
T - 4000=400a
The problem is with the rope...