Aspring with a spring constant of 200n/m is compressed 15cm when a mass is placed on top of it. what is this mass?

The weigth of the mass placed on top of the spring provides the force that compresses it:

where m is the mass and g is the gravitational acceleration.

For Hook's law, the compression  of the spring is related to the force applied by 

where k is the spring constant. Using  and k=200 N/m, we can equalize the two forces to find the mass m: