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It is because if you look at the the information in the question given then it states that at t=0 the mass was released....so it was the point when the resultant force does infact exist and it could have been 0 resultant force if the mass was released in any other time then t=0.b (iii)
why isn't the mass in equilibrium at t=0? Since the weight + force applied to pull the mass = tension in the spring? the resultant force is zero.