What is the acceleration of this object? the object's mass is 60 kg.

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