First determine the net force. Let's say the downwards force is negative and the upwards force is positive.
Since the forces act in opposite directions, the net force would be:
400N - 600N = -200N
Since I said negative is downwards, this translates to the net force being 200N downwards.
Force = mass*acceleration
200N = 60kg * acceleration
acceleration = 3.33 m/s^2
