In this case we are given a force of 29,000 N. We know that the equation we can use is:
Force = mass * acceleration
However this is not just any type of force, because this is the weight of the elevator. So when the force is equal to weight, the acceleration acting is simply gravity. Therefore:
weight = mass * gravity
Calculating for mass:
mass = 29,000 N / (9.8 m/s^2)
mass = 2,959.18 kg