Hooke's law describes a certain light spring of unstretched length 33.6 cm. when one end is attached to the top of a doorframe and a 6.89 kg object is hung from the other end, the length of the spring is 43.2 cm.

Missing question: "What is the spring's constant?"

Solution:
The object of mass m=6.89 kg exerts a force on the spring equal to its weight:

When the object is attached to the spring, the displacement of the spring with respect to its equilibrium position is

And by using Hook's law, we can find the constant of the spring: