Solve the all parts of given problem:
Problem: A person stands on a bathroom scale in an elevator that is accelerating upward. The scale reads a weight of 836.4. The same person takes a second reading while holding a box of mass 21.7 kg; the scale reads a weight of 1058.1 N.
Part A: What is the person's mass?
Part B: What is the acceleration of the elevator?
Please show each step to compute the acceleration of the elevator.