Hello thereddevilsThe technical answer is that you need to find the difference between the vectors representing the two accelerations. So if we take the direction vertically downwards as positive, then the acceleration of the lift is , and . So the difference is .
The more intuitive (and simpler) answer is to ask yourself: do you feel heavier or lighter when a lift is accelerating upwards? The answer, of course, is heavier. So the acceleration of the lift is added to the acceleration due to gravity.
Grandad