If you mean the z-scores of 3.9-3.5, no this is the z-score difference which doesn't really have any significance. We want to find the probability between these z-scores which we do use the probability table. Here, the z-scores are 2.26 and 0.97 respectively. Hence we need to find P(2.26)-P(0.97) Using the table, you simply find the corresponding area value of 2.26 (from the table) and subtract the area value of 0.97 (again from the table).

If you mean the z-scores of 3.9-3.5, no this is the z-score difference which doesn't really have any significance. We want to find the probability between these z-scores which we do use the probability table. Here, the z-scores are 2.26 and 0.97 respectively. Hence we need to find P(2.26)-P(0.97) Using the table, you simply find the corresponding area value of 2.26 (from the table) and subtract the area value of 0.97 (again from the table).